cpu/arm/k60: Change default debug level to -g3

Added a few comments about the debugging options in CFLAGS in the
cpu Makefile.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add -fno-common to CFLAGS.

-fno-common avoids accidentally using the same variable in different
files by disabling common symbols.

When a variable is supposed to be used from multiple files it should be
declared extern in the header file and only have one definition (in a
.c-file), with or without an initial value.

See also:
 - https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html
 - http://permalink.gmane.org/gmane.comp.gcc.help/41650

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Clean up warnings in 1-wire driver.

- Corrects a few missing type casts which were not caught before.
- Add missing declaration of ow_skip_or_match_rom() to onewire.h.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>