SAM4L: Add logic to enable selected peripherals on power up; Extend configuration so that each peripheral can be selected -- even though the drivers are not yet implemented

Fix a backward conditional that cause the STM32 usb host driver to fail to detect disconnection events.  From Scott

Add flow control support to the STM32 serial driver; Fix some issues with UART2 and 5.  From Lorenz Meier and Mike Smith

Add register definitions for SAM4L BPM and SCIF blocks; SAM4L clock initialization now selects an optimal  power scaling mode

If CONFIG_STM32_DMACAPABLE is defined, use stm32_dmacapable to workaround attempt SPI DMAs from the CCM stack

Add missing NSH configuration settings.  Correct some conditional logic for STM32 FALSH pre-fetch settings.  From Lorenz Meier

Eliminated sam3u_internal.h. Use separate header files instead.  More renaming from sam3u_ to sam_ to make room in the namespce for the sam4l_

Move SAM3U header files to arch/arm/src/sam3u/chip. Some renaming of SAM3U to SAM to ssupport SAM4.  Convert all configs/sam3u-ek configurations to use the kconfig-frontends tool

Upate .gitignore files.  Add .dSYM. Make sure / is present where needed.  Add some missing .gitignore files

Optimize request buffer sizes in the serial USB drivers so that we avoid so many NULL packets and so that the OUT request buffer size is exactly the max packet size.