onie-syseeprom: Get erase length of MTD device from kernel

The erase length for sys_eeprom is passed from machine's busybox
config.  It is not clever enough when the machine supports 2 or
more NOR flash with different sector/erase size.  Instead of passing
erase length from the config, onie-syseeprom can catch it from linux
kernel at runtime directly.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Accton powerpc platforms: Remove erase size config from busybox

The platforms are including:
- AS4600_54T
- AS5600_52X
- AS5610_52X
- AS5710_54X
- AS6700_32X
- AS6701_32X
- AS6710_32X
- AS7700_32X

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>