[U-Boot] Allow board specific overwriting of library code

[Michael Roth]

Hello,

recently there were some discussion how to support board specfic
lowlevel_init code on the at91 plattform. For example in Nov 2008
there was a discussion about CONFIG_USER_LOWLEVEL_INIT or so and
recently there was a patch proposal from Jean-Christophe to provide
a cmd_link_o_target macro that creates a combinend object file
instead an archive.

The combined object file was needed because weak linking somehow
doesn't work with archive files. Very annoying. 

I had large problems with weak linking regarding coloured LED support
on my board, too. I came to the conclusion that weak linking across
archives are somehow broken. However I was not able to create a
small test case to trigger this behavior (bug?) in gcc/bintuils.

Ok, so here my proposal that is generic and allows any library code
to get overwritten by board specific functions without defining any
weak symbols in the library code. It is based purely on sequence of
object files presented to 'ld'.

Please take a look and tell me your opinion.

I have patches to support a new board in my queue but I need any
possibility to provide a board specific lowlevel_init. Which solution
is selected plays no role to me.


Michael Roth