imx: Put flash header position specific sections back in

[Marc Reilly]

Hi,

This patch adds the position specific flash_header_0xXXXX section defines 
(and friends) so the one barebox image can support multiple boot sources, 
for example Nor and SD. 

If a board only uses one boot source, the existing generic section defines
are simpler to use, so I've left them.

Cheers
Marc