support/scripts/apply-patches: fallback $TAR to `tar`

Commit f77d698f83b230f7efcb7d28af3aacfa9589cc89 changed apply-patches to
use $TAR instead of `tar`, but did not define a fallback if $TAR is not
defined. This results in an error when calling apply-patches.sh from
outside Buildroot's Makefile. Our team uses this script to setup local
checkouts of Buildroot package's with patches for development.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/scripts/apply-patches.sh

@@ -46,6 +46,8 @@ patchdir=${2-../kernel-patches}
 shift 2
 patchpattern=${@-*}
 
+export TAR=${TAR:-tar}
+
 # use a well defined sorting order
 export LC_COLLATE=C