Change platform detect to use /proc/cpuinfo to detect board type and add new board to detect.

[Groguard]

This PR changes how boards are detected by using "/proc/cpuinfo" instead of reading the platform. This change checks for the processor type and determines the board based on that. This should allow for more boards to be added and detected for use with adafruit-blinka.

Changes:

• added 'import os'
• removed all extra beaglebone elif's as they aren't needed anymore
• added GIANT_BOARD to the list of boards.

limitations:

• Boards with the same processor will be indistinguishable

Test code:

import os

# Platform identification constants.
UNKNOWN          = 0
RASPBERRY_PI     = 1
BEAGLEBONE_BLACK = 2
MINNOWBOARD      = 3
GIANT_BOARD      = 4

command = 'cat /proc/cpuinfo'
plat = os.popen(command).read().strip()

def return_platform():
    if plat.lower().find('generic am33xx') > -1:
        return BEAGLEBONE_BLACK
    elif plat.lower().find('atmel sama5') > -1:
        return GIANT_BOARD

print(return_platform())

[ladyada]

@Groguard ooh thanks - we're actually about to pull this out and into a separate repo just for platform detection. @brennen is doing that now, and may ask you to add your code there!