vboot_display: fix -Wwrite-strings compiler errors

VbHWID() returns a fixed string which can also be a returning a
string that lives in the read only text segment or read only flash.
Because of this the compiler will rightfully error when built with
-Werror and -Wwrite-strings because the const qualifier is being
ignored.  Correct this and the ripple effect in VbRenderTextAtPos().
VbRenderTextAtPos() should be using constant strings anyhow.

BUG=chrome-os-partner:17695
BRANCH=None
TEST=built fwlib with -Werror -Wwrite-strings. Compiles correctly. make
     runtests still passes as well.

Change-Id: Id03a783d2d9b5de4cc3b3ce0fa6d74934751cca4
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/44043
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed