gen_initramfs.sh: use absolute path for gen_init_cpio

Use absolute path to call gen_init_cpio. This allows one
to use gen_initramfs.sh from any directory.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>

usr/gen_initramfs.sh

@@ -244,4 +244,4 @@ if test -n "$KBUILD_BUILD_TIMESTAMP"; then
 		timestamp="-t $timestamp"
 	fi
 fi
-usr/gen_init_cpio $timestamp $cpio_list > $output
+"$(dirname "$0")"/gen_init_cpio $timestamp $cpio_list > $output