kbuild: add OBJSIZE variable for the size tool

commit 7bac98707f65b93bf994ef4e99b1eb9e7dbb9c32 upstream.

Define and export OBJSIZE variable for "size" tool from binutils to be
used in architecture specific Makefiles (naming the variable just "SIZE"
would be too risky). In particular this tool is useful to perform checks
that early boot code is not using bss section (which might have not been
zeroed yet or intersects with initrd or other files boot loader might
have put right after the linux kernel).

Link: http://lkml.kernel.org/r/patch-1.thread-2257a1.git-188f5a3d81d5.your-ad-here.call-01565088755-ext-5120@work.hours


Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
[nd: conflict in exported vars list from not backporting commit
 e83b9f55448a ("kbuild: add ability to generate BTF type info for vmlinux")]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>