faft: Add methods to backup and restore kernel

To make sure the kernel does not get changed when running a test, we
need methods to backup and restore the kernel partition.

BUG=chromium:215787
TEST=zero out kernel B and run firmware_CorruptKernelA. Check kernel B
is all zero after the test.

Change-Id: I80d4226416c53b952dc9550d045780d048bb5cef
Signed-off-by: Vic (Chun-Ju) Yang <victoryang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/185320
Reviewed-by: Yusuf Mohsinally <mohsinally@chromium.org>
3 files changed