Provide method for erased flash to have a valid ECC.

This is so that the produced ECC can match the software ECC for systems that
do this (like Linux). Its also useful for software that checks for erased flash
by comparing it to 0xff.
Fix bch_defs comment typo
Fix typo in serial_mixed_multiplier preventing compilation of bch_error_bma_serial
Fix typo in const_matrix_multiply effecting non-rectangular matrixes

Luckily we don't have any of these.
Handle const cases where entire rows are empty.

This was breaking M=4.