board: ti: common: Fix pointer-bool-conversion warnings

When building this code with clang-10 a number of warnings will be
generated along the lines of:
warning: address of array 'ep->version' will always evaluate to 'true'

Convert these checks to checking the strlen of the part of the array we
care about.  As this array will be null terminated previously by us,
this is safe.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>