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>
diff --git a/scripts/headers.sh b/scripts/headers.sh
index 1ddcdd3..978b42b 100755
--- a/scripts/headers.sh
+++ b/scripts/headers.sh
@@ -13,7 +13,7 @@
 	fi
 }
 
-archs=$(ls ${srctree}/arch)
+archs=${HDR_ARCH_LIST:-$(ls ${srctree}/arch)}
 
 for arch in ${archs}; do
 	case ${arch} in