Make Msp430ClockC a generic so that the target DCO clock and the ACLK can be

Make Msp430ClockC a generic so that the target DCO clock and the ACLK can be
passed in as instantiation variables.  This provides a more TinyOS savvy
mechanism to set desired clocks as opposed to the prior method which involved
overriding macros defined in Msp430DcoSpec.h.  This and other enhancements to
the clock initialization has also slightly reduced executable sizes.  Below
is the Blink application from TinyOS 2.1.0, compiled with the plain vanilla
2.1.0 and with the clock init modifications for all msp430 platforms in the
standard distribution:

Compiled ROM sizes in bytes

|| Platform  || 2.1.0 || updated ||
||           ||       ||         ||
|| eyesIFX   || 2570  || 2510    ||
|| shimmer   || 2522  || 2462    ||
|| telosa    || 2538  || 2478    ||
|| telosb    || 2634  || 2574    ||
|| tinynode  || 2896  || 2838    ||