scripts: extract_dts_includes.py: Correct LOAD_OFFSET

This patch fixes a bug with the computation of the FLASH_LOAD_OFFSET
option that is derived as part of the DTS parsing.  Offset should be
the relative offset from the base of flash.  If the flash base address
and selected partition are the same, the offset should be 0.

JIRA: ZEP-2260

Signed-off-by: Andy Gross <andy.gross@linaro.org>