generic: fix occasional "execvp: /bin/sh: Argument list too long" is kernel header install phase (#12907)

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38998 3c298f89-4303-0410-b956-a3cf2f4a3e73