Especially when using #include directives in imxcfg files, error
messages can be hard to follow because line numbers differ between
preprocessor and source files. Teach the imx image generation tool about
preprocessing line number / file name directive, so it can give more
useful error messages. Instead of:

  error in line 11: Invalid argument

It now says:

  $barebox/arch/arm/boards/$som/flash-header-$board.imxcfg:5: Invalid argument

No change in compiled barebox binaries.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>