toolchain: add more K20 series targets

To allow them all to use the same startup code and such I replaced the
hardcoded ifeq in toolchain/lib/mchck/Makefile.part with a check on a
new variable called TARGET_FAMILY. It's set to 'K20' in all the target
.mk files to show they're all using the K20 code.
Merge remote-tracking branch 'upstream/master' into i2c