ti: common: board_detect: Return a valid empty string for un-initialized eeprom

ti: common: board_detect: Return a valid empty string for un-initialized eeprom

Current logic for query of revision, board_name, config returns
NULL. Users of these functions do a direct strncmp to compare.
Unfortunately, as per conventions require two valid strings to compare
against and the current implementation causes a crash when compared
with NULL.

We'd still like to maintain the simplistic usage of these APIs instead
of redundant if (string) res=strncmp(fn(),"cmp",n); flowing all over
the place.

Hence, since the version, name and config is already pre-initialized
with empty string, just dont check for invalid header in the first
place and return the empty string to the caller.

Reported-by: Brad Griffis <bgriffis@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Correct was'nt -> wasn't typo]
Signed-off-by: Tom Rini <trini@konsulko.com>