vboot_reference/tests: Extend ShaNi test to arm platforms

The subtest method vb2ex_hwcrypto_digest_extend in vb2_sha256_x86_tests
is defined for arm based boards too. If we add these to the Makefile for
arm platforms then we can have this test common for both x86 and arm.

Additionally, this patch adds a check for whether the device supports
SHA256 HW acceleration for arm platforms, similar to the x86 check.

This patch also renames the test removing x86 from the name.

BUG=483238287
TEST=ShaNi test on qualcomm mica

Change-Id: I6bbab890b420b522b101277ce699d3473c41f01c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/8559757
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>
Commit-Queue: Avi Uday <aviuday@google.com>
Tested-by: Avi Uday <aviuday@google.com>
Reviewed-by: Kshitij Shah <tij@google.com>