platform/mulle/mk60/Makefile.mk60: Clean up toolchain definitions

Added conditionals to allow overriding the defaults for the GCC path
and target triple.

Usage:
export GCCBIN=/path/to/your/toolchain/gcc/bin
make TARGET=mulle

Additionally, if you would like to build using another target triple:
export TARGET_TRIPLE=armv7m-none-eabi-or-whatever
make TARGET=mulle

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>