tools: imx_header should not include flash_offset

tools: imx_header should not include flash_offset

Doing a  make distclean; make mx6qsabresd_config; make
and      hexdump -C u-boot.imx | less

 ...
 00000360  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
 *
 000003f0  00 00 00 00 00 00 00 00  00 00 00 00 00 04 00 00  |................|
                                                ^^^^^^^^^^^
 00000400  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
 *
 00001000  13 00 00 ea 14 f0 9f e5  14 f0 9f e5 14 f0 9f e5  |...ê.ð.å.ð.å.ð.å|
 ...

shows the flash_offset value being written into the final
generated image, wich is not correct.

Instead create flash_offset as static variable such that the
generated image is "clean".

 00000360  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
 *
 00001000  13 00 00 ea 14 f0 9f e5  14 f0 9f e5 14 f0 9f e5  |...ê.ð.å.ð.å.ð.å|

Signed-off-by: Stefano Babic <sbabic@denx.de>