cpu/arm/k60: Implement TX buffering in UART1.

Uses a ring buffer of configurable size and TX interrupts.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: SystemCoreClockUpdate readability.

Replaced magic numbers with symbolic macros.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Build fixes for Clang+Newlib.

Clang needs to be told where to look for Newlib includes, this is not
necessary on GCC where the cross compiler is usually installed in a
separate directory structure with specified include directories.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add missing header in devoptab.h

reent.h needed for struct _reent

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Correct types on syscall declarations.

Adds some const correctness and different return types on the syscalls
that caused build errors on Clang. Now matches newlib syscall definitions.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add more debug output during boot.

 - Adds a printout for system frequencies.
 - Replaced printf() with PRINTF() to match other parts of the file.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: WIP UART TX buffering.

This is a work in progress, currently seem to work fine for UART1.
Hard coded to always use UART1 for some RX things.

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