pepper: Implement Board Detection mechanism

AM335x-based 'Gumstix Pepper' SBCs and variants use different types of
RAM (DDR2 vs DDR3 with DDR3 being the default).  Detect the board type
by reading the factory-programmed EEPROM [1] and use this to select any
runtime boot options such as RAM type.

[1] http://elinux.org/BeagleBoardPinMux#List_of_Vendor_and_Device_IDs



Signed-off-by: Adam YH Lee <adam.yh.lee@gmail.com>
Signed-off-by: Ash Charles <ashcharles@gmail.com>