imx: Internal boot sources, handle all header offsets
Makes the internal boot source configurable.
Also changes section names slightly so that .flash_header_0x1000 isn't
matched to .flash_header_0x100* etc.
Signed-off-by: Marc Reilly <marc@cpdesign.com.au>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>