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 <kirill@shutemov.name>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/make/headers_install.txt

@@ -39,8 +39,9 @@ INSTALL_HDR_PATH indicates where to install the headers.  It defaults to
 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
 <dwmw2@infradead.org>.

scripts/headers.sh

@@ -13,7 +13,7 @@ do_command()
 	fi
 }
 
-archs=$(ls ${srctree}/arch)
+archs=${HDR_ARCH_LIST:-$(ls ${srctree}/arch)}
 
 for arch in ${archs}; do
 	case ${arch} in