Mecrisp Lowpower for G2553

For saving power, this experimental release for MSP430G2553 runs 
its communication with a 32768 Hz ACLK crystal and in LPM3.

One example to use lowpower modes to wait:

: delayticks ( ticks -- ) \ Wait desired number of ticks with 32768 Hz.

  ['] wakeup irq-timerb0 ! \ Set Wakeup-Interrupt
  $10  $0182 ! \ Enable Interrupt
  0    $0190 ! \ Clear TAR
       $0192 ! \ Set desired delay
  $110 $0180 ! \ ACLK, Up-Mode
  lpm3         \ Sleep in LPM3
  0 $180 !     \ Stop Timer
;

Interrupts are enabled at the beginning.

You can compare power consumption with normal operation by 
disabling interrupts with DINT and reenable them with EINT while
measuring current. Terminal routines can work in both modes.

Matthias Koch, Summer 2013
