kbuild: replace genhdr-y with generated-y

Originally, generated-y and genhdr-y had different meaning, like
follows:

- generated-y: generated headers (other than asm-generic wrappers)
- header-y   : headers to be exported
- genhdr-y   : generated headers to be exported (generated-y + header-y)

Since commit fcc8487d477a ("uapi: export all headers under uapi
directories"), headers under UAPI directories are all exported.
So, there is no more difference between generated-y and genhdr-y.

We see two users of genhdr-y, arch/{arm,x86}/include/uapi/asm/Kbuild.
They generate some headers in arch/{arm,x86}/include/generated/uapi/asm
directories, which are obviously exported.

Replace them with generated-y, and abolish genhdr-y.

Change-Id: I9b8b97c39673e93e3d1215090f31d68beffddbd7
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Git-Commit: ae3f4151737d2aad271e5b1f2553c5d56e52850c
Git-Repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[bharad@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Naitik Bharadiya <bharad@codeaurora.org>