kbuild: rebuild initramfs if content of initramfs changes

initramfs.cpio.gz being build in usr/ and included in the
kernel was not rebuild when the included files changed.

To fix this the following was done:
- let gen_initramfs.sh generate a list of files and directories included
  in the initramfs
- gen_initramfs generate the gzipped cpio archive so we could simplify
  the kbuild file (Makefile)
- utilising the kbuild infrastructure so when uid/gid root mapping changes
  the initramfs will be rebuild

With this change we have a much more robust initramfs generation.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

usr/Makefile

@@ -1,65 +1,48 @@
+#
+# kbuild file for usr/ - including initramfs image
+#
 
+klibcdirs:;
+
+# Generate builtin.o based on initramfs_data.o
 obj-y := initramfs_data.o
 
-hostprogs-y  := gen_init_cpio
-
-clean-files := initramfs_data.cpio.gz initramfs_list
-
 # initramfs_data.o contains the initramfs_data.cpio.gz image.
 # The image is included using .incbin, a dependency which is not
 # tracked automatically.
 $(obj)/initramfs_data.o: $(obj)/initramfs_data.cpio.gz FORCE
 
-ifdef CONFIG_INITRAMFS_ROOT_UID
-gen_initramfs_args += -u $(CONFIG_INITRAMFS_ROOT_UID)
+#####
+# Generate the initramfs cpio archive
+
+hostprogs-y := gen_init_cpio
+initramfs   := $(CONFIG_SHELL) $(srctree)/scripts/gen_initramfs_list.sh
+ramfs-input := $(if $(filter-out "",$(CONFIG_INITRAMFS_SOURCE)), \
+                    $(CONFIG_INITRAMFS_SOURCE),-d)
+ramfs-args  := \
+        $(if $(CONFIG_INITRAMFS_ROOT_UID), -u $(CONFIG_INITRAMFS_ROOT_UID)) \
+        $(if $(CONFIG_INITRAMFS_ROOT_GID), -g $(CONFIG_INITRAMFS_ROOT_GID)) \
+        $(ramfs-input)
+
+# .initramfs_data.cpio.gz.d is used to identify all files included
+# in initramfs and to detect if any files are added/removed.
+# Removed files are identified by directory timestamp being updated
+# The dependency list is generated by gen_initramfs.sh -l
+ifneq ($(wildcard $(obj)/.initramfs_data.cpio.gz.d),)
+	include $(obj)/.initramfs_data.cpio.gz.d
 endif
 
-ifdef CONFIG_INITRAMFS_ROOT_GID
-gen_initramfs_args += -g $(CONFIG_INITRAMFS_ROOT_GID)
-endif
+quiet_cmd_initfs = GEN     $@
+      cmd_initfs = $(initramfs) -o $@ $(ramfs-args) $(ramfs-input)
 
-# The $(shell echo $(CONFIG_INITRAMFS_SOURCE)) is to remove the
-# gratuitous begin and end quotes from the Kconfig string type.
-# Internal, escaped quotes in the Kconfig string will loose the
-# escape and become active quotes.
-quotefixed_initramfs_source := $(shell echo $(CONFIG_INITRAMFS_SOURCE))
-
-filechk_initramfs_list = $(CONFIG_SHELL) \
- $(srctree)/scripts/gen_initramfs_list.sh $(gen_initramfs_args) $(quotefixed_initramfs_source)
-
-$(obj)/initramfs_list: $(obj)/Makefile FORCE
-	$(call filechk,initramfs_list)
-
-quiet_cmd_cpio = CPIO    $@
-      cmd_cpio = ./$< $(obj)/initramfs_list > $@
-
-
-# Check if the INITRAMFS_SOURCE is a cpio archive
-ifneq (,$(findstring .cpio,$(quotefixed_initramfs_source)))
-
-# INITRAMFS_SOURCE has a cpio archive - verify that it's a single file
-ifneq (1,$(words $(quotefixed_initramfs_source)))
-$(error Only a single file may be specified in CONFIG_INITRAMFS_SOURCE (="$(quotefixed_initramfs_source)") when a cpio archive is directly specified.)
-endif
-# Now use the cpio archive directly
-initramfs_data_cpio = $(quotefixed_initramfs_source)
-targets += $(quotefixed_initramfs_source)
-
-else
-
-# INITRAMFS_SOURCE is not a cpio archive - create one
-$(obj)/initramfs_data.cpio: $(obj)/gen_init_cpio \
-                            $(initramfs-y) $(obj)/initramfs_list FORCE
-	$(call if_changed,cpio)
-
-targets += initramfs_data.cpio
-initramfs_data_cpio = $(obj)/initramfs_data.cpio
-
-endif
-
-
-$(obj)/initramfs_data.cpio.gz: $(initramfs_data_cpio) FORCE
-	$(call if_changed,gzip)
-
-targets += initramfs_data.cpio.gz
+targets := initramfs_data.cpio.gz
+$(deps_initramfs): klibcdirs
+# We rebuild initramfs_data.cpio.gz if:
+# 1) Any included file is newer then initramfs_data.cpio.gz
+# 2) There are changes in which files are included (added or deleted)
+# 3) If gen_init_cpio are newer than initramfs_data.cpio.gz
+# 4) arguments to gen_initramfs.sh changes
+$(obj)/initramfs_data.cpio.gz: $(obj)/gen_init_cpio $(deps_initramfs) klibcdirs
+	$(Q)$(initramfs) -l $(ramfs-input) > $(obj)/.initramfs_data.cpio.gz.d
+	$(call if_changed,initfs)