Bootloader LEDs / TXRX activity / USB Descriptors / Makefile

[cmaglie]

This is a rebase and merge of:
#122 Bootloader LED pulsing and TX/RX blinking
#104 Enable USB string descriptors
#69 Add Leonardo-like led pulse when bootloader is active
#64 Bootloader leds activity

Moreover:

• each one of the PR above added a feature that is board-specific (LEDs pins, USB descriptors, etc.) so instead of adding more variables to pass to compiler I've changed the build process so each board can be defined through his own board_descriptions_xxxx.h file.
• the TX/RX leds and on-board LED can now be disabled
• I've added a script to build all the bootloaders altogether

Tested with Arduino Zero and MKR1000 on Linux.

Fix #1

/cc @aethaniel @facchinm @sandeepmistry

[sandeepmistry]

Looks good on OS X 10.11.6 + IDE 1.6.12 + Arduino Zero (Native USB)

• LED pulses in boot loader mode
• double tap via USB puts board in boot loader mode
• RX and TX LED's blink on sketch upload
• OS X shows USB string descriptors

[sandeepmistry]

Looks good on my Windows 10 VM as well. I think we should still test with a non-VM windows PC though.