* Add boot2 feature. Add boot2 linkage into each BSP optional on feature
* Enable boot2 feature in BSPs by default. Remove boot2 decl from all BSP examples
* Add EXTERN in memory.x for BOOT2_FIRMWARE, rename bootloader static slice to BOOT2_FIRMWARE
* Update new examples and itsy_bitsy BSP to use boot2 feature
* Remove boot2 as a dev-dependency for the BSPs, no longer needed
* Add no_mangle BOOT2_FIRMWARE to adafruit_macropad
* Fix itsy-bitsy blinky - it wasn't using the BSP, so it didn't get BOOT2_FIRMWARE linked in
Answers on a postcard please on how to avoid `static mut` when you have
lazy initialization, and static objects holding references to other
static objects.
Set interrupt enable flags in USB driver. The flags were already being cleared in the USB driver.
Update USB examples to remove interrupt flag set/check as it is no longer necessary.
