kbuild: rename built-in.o to built-in.a

Incremental linking is gone, so rename built-in.o to built-in.a, which
is the usual extension for archive files.

This patch does two things, first is a simple search/replace:

git grep -l 'built-in\.o' | xargs sed -i 's/built-in\.o/built-in\.a/g'

The second is to invert nesting of nested text manipulations to avoid
filtering built-in.a out from libs-y2:

-libs-y2 := $(filter-out %.a, $(patsubst %/, %/built-in.a, $(libs-y)))
+libs-y2 := $(patsubst %/, %/built-in.a, $(filter-out %.a, $(libs-y)))

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -35,7 +35,7 @@ unexport GREP_OPTIONS
 # Most importantly: sub-Makefiles should only ever modify files in
 # their own directory. If in some directory we have a dependency on
 # a file in another dir (which doesn't happen often, but it's often
-# unavoidable when linking the built-in.o targets which finally
+# unavoidable when linking the built-in.a targets which finally
 # turn into vmlinux), we will call a sub make in that other dir, and
 # after that we are sure that everything which is in that other dir
 # is now up to date.
@@ -982,13 +982,13 @@ vmlinux-dirs	:= $(patsubst %/,%,$(filter %/, $(init-y) $(init-m) \
 vmlinux-alldirs	:= $(sort $(vmlinux-dirs) $(patsubst %/,%,$(filter %/, \
 		     $(init-) $(core-) $(drivers-) $(net-) $(libs-) $(virt-))))
 
-init-y		:= $(patsubst %/, %/built-in.o, $(init-y))
-core-y		:= $(patsubst %/, %/built-in.o, $(core-y))
-drivers-y	:= $(patsubst %/, %/built-in.o, $(drivers-y))
-net-y		:= $(patsubst %/, %/built-in.o, $(net-y))
+init-y		:= $(patsubst %/, %/built-in.a, $(init-y))
+core-y		:= $(patsubst %/, %/built-in.a, $(core-y))
+drivers-y	:= $(patsubst %/, %/built-in.a, $(drivers-y))
+net-y		:= $(patsubst %/, %/built-in.a, $(net-y))
 libs-y1		:= $(patsubst %/, %/lib.a, $(libs-y))
-libs-y2		:= $(filter-out %.a, $(patsubst %/, %/built-in.o, $(libs-y)))
-virt-y		:= $(patsubst %/, %/built-in.o, $(virt-y))
+libs-y2		:= $(patsubst %/, %/built-in.a, $(filter-out %.a, $(libs-y)))
+virt-y		:= $(patsubst %/, %/built-in.a, $(virt-y))
 
 # Externally visible symbols (used by link-vmlinux.sh)
 export KBUILD_VMLINUX_INIT := $(head-y) $(init-y)