Platform M2351: Add config.cmake

Use new style RTX link.
Add config.cmake and force MCUBOOT_IMAGE_NUMBER=1
Update README.rst to fix typo and remove MCUBOOT_IMAGE_NUMBER=1 in
command line.

Signed-off-by: WSChang <wschang0@nuvoton.com>

Change-Id: I9b3e646dd222938ac614475456b33970cef863d9
4 files changed