toolchain/mips: kill EABI and fix N32

MIPS EABI is a bare-metal ABI so remove it.
Also fix uClibc to really work with N32 ABI, which used the EABI knob
previously.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>