Implement cgptlib for MTD devices.

Defines MTD on-disk structures & API in mtdlib.h/c that closely mirrors
the existing cgpt implementation. Currently, the disk structures do not
contain guids or labels, and the number of available partition types and
quantities are limited, exactly what we want to support should be decided
before we ship this.

Adds appropriate test coverage to the unit test library - either by modifying
existing tests, or copying them and changing them accordingly.

BUG=chromium:221745
TEST=added appropriate tests to the unittests
BRANCH=none

Change-Id: Iee19864498024c72229bc3c7811594fe762f52de
Original-Change-Id: I031eca69d6c8e825b02bd0522d57e92b05eb191a
Reviewed-on: https://gerrit.chromium.org/gerrit/46082
Tested-by: Albert Chaulk <achaulk@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48793
Reviewed-by: Albert Chaulk <achaulk@chromium.org>

firmware/lib/cgptlib/include/cgptlib.h

@@ -21,6 +21,8 @@ enum {
 	GPT_ERROR_START_LBA_OVERLAP,
 	GPT_ERROR_END_LBA_OVERLAP,
 	GPT_ERROR_DUP_GUID,
+	GPT_ERROR_INVALID_FLASH_GEOMETRY,
+	GPT_ERROR_NO_SUCH_ENTRY,
 	/* Number of errors */
 	GPT_ERROR_COUNT
 };