Updates bootloader settings, adds file size check

[jackhumbert]

This fixes the Teensy reset issue from #164, and speeds up the DFU reset time by using PJRC's same USB disable and delay in that chain.

This introduces the BOOTLOADER variable into rules.mk files (now overridable) - this is the standard block:

# Bootloader
#     This definition is optional, and if your keyboard supports multiple bootloaders of
#     different sizes, comment this out, and the correct address will be loaded 
#     automatically (+60). See bootloader.mk for all options.
BOOTLOADER = atmel-dfu

If it's possible that multiple bootloaders can be used for one project of the same revision, you can leave this unset, and the correct size will be selected automatically (this is new and +60 bytes). The current options for BOOTLOADER = are:

• atmel-dfu
• lufa-dfu
• qmk-dfu
• halfkay
• caterina
• bootloadHID

BOOTLOADER_SIZE can still be defined manually, but if I'm missing anything (that's currently being used) please let me know and I'll add it to the list!

If different revisions of a project use different bootloaders, you can use a block like this to define a bootloader for each (this can also be done by separate rules.mk files):

# Bootloader
#     This definition is optional, and if your keyboard supports multiple bootloaders of
#     different sizes, comment this out, and the correct address will be loaded 
#     automatically (+60). See bootloader.mk for all options.
ifeq ($(strip $(KEYBOARD)), preonic/rev1)
    BOOTLOADER = atmel-dfu
endif
ifeq ($(strip $(KEYBOARD)), preonic/rev2)
    BOOTLOADER = qmk-dfu
endif

File size check

This also introduces the target check-size, that automatically figures out how much flashable space a keyboard will have with a certain bootloader (using the settings above) - it's called automatically, and you'll be able to see how many bytes you have left when compiling (in decimal). Currently this is only available for AVR chips.

DOTOO

• update all the keyboard's rules.mk and get rid of (now) unnecessary defines.
• document all of this in docs/