ext-toolchain: MIPS: Select correct endianness for the target

Previously, an external-toolchain on a mipsel/mips64el target
didn't select the appriopriate endianness but
it asked the user to set the correct CFLAGS on his/her own.
We fix this by appending "-EL" to the toolchain wrapper options
if the user has selected a mipsel/mips64el target.

[Thomas: remove unneeded test on BR2_ENDIAN, since mipsel and mips64el
are always little-endian, and add the corresponding big endian case.]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>