MIPS: remove OUTPUT_FORMAT from linker scripts

The OUTPUT_FORMAT command in linker scripts
was always misused due to some endianess and
toolchain problems.

Use GCC flags to ensure proper output format,
and get rid of the OUTPUT_FORMAT commands in
the board specific u-boot.lds files.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Xiangfu Liu <xiangfu@openmobilefree.net>