lib21: always append '\0' in vb2_readfile

For utilities reading text file, it is easier to process as ASCIIZ input
if vb2_readfile can always return a buffer ends with '\0' so we don't
need to pass and check the size.

BUG=None
TEST=make clean && make runtests
BRANCH=None

Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Change-Id: Ib6294969fb325b9b7899e6295fb1817ad91a9952
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2426092
Reviewed-by: Joel Kitching <kitching@chromium.org>
2 files changed