newlib/sbrk: document provided implementations and override process

The behavior of the default implentations is worth having in the
generated documentation.
doc: add msp430-elf documentation
doc/releases: update for new release
unittests/uptimescale: add missing dependency
examples: add missing FRAM platform support
examples: error where CCACLK required and not available
examples: use alternative clock when CCACLK not available

Classically we use CCACLK whenever we need a timer peripheral that's not
devoted to uptime or other critical system uses.  exp430fr4133 doesn't
have something that provides ACLK, so use an explicit alternative timer.
dma: use long int instead of mspgcc-specific uint20_t

msp430-elf doesn't define a 20-bit type that's available outside of
large memory model.  TI's headers appear to rely on unsigned long for
such addresses, so go ahead and use that.
examples: convert pointer difference to int for msp430-elf large

The difference between two pointers in -mlarge for msp430-elf is an
__int20 which is incompatible with the %d/%u format specifier.
cleanup: fix doxygen warnings
Update version for release