Merged in david_alessio/nuttx/refactor-pll-setup (pull request #581)

Refactor pll setup

* fix typo in #def

* refactor PLL setup code...

* refactored PLL/CLK config, easier, checks for correctness

* call go_os_start if STACK_COLORIZED

* smarter config of EXTCLK output freq

* cosmetic

Approved-by: Gregory Nutt <gnutt@nuttx.org>