SAML21: Add logic to support GLCK peripheral channels; add logic to configure FDPLL lock timer and ref clock GCLKs

STM32: Building stm32_rng.c should be conditioned on CONFIG_STM32_RNG, not CONFIG_DEV_RANDOM.  From Juha Niskanen

SAM3/4 Serial:  In NONBLOCKING mode serial does not receive bytes from UART.  This happens because RX interrupt was disabled in the setup routine.  To fix this I save interrupt configuration before uart shutdown and restore it after.  From Max Neklyudov.

SAML21-Xplained:  Update the cloned SAMD20-Xplained board configuration for specifics of the SAML21-Xplained

SAMD/L: Hide differences in the way peripheral clocking is enabled behind macros.  Some updates to saml_clockconfig.h + various fixes while working toward SAML21 build

Clock configuration logic for the SAML21.  Initial commit is just the SAMD20 clock configuration with some naming changes and does not compile

Add basic board build configurat for the SAML21 Xplained.  Initial commit is jsut the SAMD20 Xplained with name changes and does not yet build

SAMD/L: Move content of chip/sam_memorymap.h to chip.h; move chip/sam_pinmap. to sam_pinmap.h for consistency with other header files