hwconfig: Fix dummy initialization of {board, cpu}_hwconfig

Since board_hwconfig & cpu_hwconfig are defined as weak and dont have a
default value they will get put into the BSS if they aren't defined
elsewhere.  This is problematic as we try to utilize hwconfig before
we've relocated and thus BSS isn't setup.

Instead of giving dummy values in the board files that utilize this
feature, we can just initialize the variables to an empty string and
thus move them out of the BSS if they aren't defined elsewhere.

Also made board_hwconfig & cpu_hwconfig arrays to reduce size associated
with string pointers vs arrays.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>