Expose hardware EEPROM data validity

Reviewed By: scotte

Create a new sys_eeprom API, is_sys_eeprom_valid(), for determining
whether the the EEPROM binary data in hardware valid.

An external caller can use this to determine if the hardware contains
valid TLV EEPROM data.

Returns non-zero when the binary data in hardware is valid, otherwise
returns zero.

Testing:

Ran against a system with corrupted and uncorrupted EEPROM contents.
In both cases is_sys_eeprom_valid() returned the expected value.