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>

firmware/lib/include/vboot_display.h

@@ -34,7 +34,7 @@ ImageInfo *VbFindFontGlyph(VbFont_t *font, uint32_t ascii,
 /**
  * Try to display the specified text at a particular position.
  */
-void VbRenderTextAtPos(char *text, int right_to_left,
+void VbRenderTextAtPos(const char *text, int right_to_left,
 		       uint32_t x, uint32_t y, VbFont_t *font);
 
 /**
@@ -45,7 +45,7 @@ const char *RecoveryReasonString(uint8_t code);
 /**
  * Return a fixed string representing the HWID.
  */
-char *VbHWID(VbCommonParams *cparams);
+const char *VbHWID(VbCommonParams *cparams);
 
 /**
  * Get the number of localizations in the GBB bitmap data.