MIPS: fix endianess handling

Make endianess of target CPU configurable. Use the new config
option for dbau1550_el and pb1000 boards.

Adapt linking of standalone applications to pass through
endianess options to LD.

Build tested with:
 - ELDK 4 mips_4KC- and mips4KCle
 - Sourcery CodeBench Lite 2011.03-93

With this patch all 26 MIPS boards can be compiled now in one step by
running "MAKEALL -a mips".

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>