kbuild: introduce HDR_ARCH_LIST for headers_install_all

Using HDR_ARCH_LIST you can specify subset of architectures you want to get
headers for.

Signed-off-by: Kirill A. Shutemov <[e-mail]>
Signed-off-by: Michal Marek <[e-mail]>

Documentation/make/headers_install.txt

···
 The command "make headers_install_all" exports headers for all architectures
 simultaneously.  (This is mostly of interest to distribution maintainers,
 who create an architecture-independent tarball from the resulting include
-directory.)  Remember to provide the appropriate linux/asm directory via "mv"
-or "ln -s" before building a C library with headers exported this way.
+directory.)  You also can use HDR_ARCH_LIST to specify list of architectures.
+Remember to provide the appropriate linux/asm directory via "mv" or "ln -s"
+before building a C library with headers exported this way.
 
 The kernel header export infrastructure is maintained by David Woodhouse
 <[e-mail]>.

scripts/headers.sh

···
 	fi
 }
 
-archs=$(ls ${srctree}/arch)
+archs=${HDR_ARCH_LIST:-$(ls ${srctree}/arch)}
 
 for arch in ${archs}; do
 	case ${arch} in