futility: Add --arch=arm64 option to vbutil_kernel

vbutil_kernel already supports 64bit arm architecture,
but it just allows "aarch64" option. However other script,
for example build_kernel_image uses ARCH environment variable,
which for arm64-generic overlay is defined as "arm64".
So vbutil_kernel will refuse the call.

BUG=None
TEST=run vbutil_kernel --arch=arm64
Check that the "Unknown architecture string: arm64" is gone

BRANCH=None

Change-Id: I94c547d6b6940ab8c622a6b8cff49b5f83c1fcad
Reviewed-on: https://chromium-review.googlesource.com/1080529
Commit-Ready: Adam Kallai <kadam@inf.u-szeged.hu>
Tested-by: Adam Kallai <kadam@inf.u-szeged.hu>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

Author: kaadam

futility/cmd_vbutil_kernel.c

@@ -299,7 +299,8 @@ static int do_vbutil_kernel(int argc, char *argv[])
 			if ((!strncasecmp(optarg, "x86", 3)) ||
 			    (!strcasecmp(optarg, "amd64")))
 				arch = ARCH_X86;
-			else if ((!strcasecmp(optarg, "arm")) ||
+			/* check the first 3 characters to also detect arm64 */
+			else if ((!strncasecmp(optarg, "arm", 3)) ||
 				 (!strcasecmp(optarg, "aarch64")))
 				arch = ARCH_ARM;
 			else if (!strcasecmp(optarg, "mips"))