Exclude predefined variables from board config

mk2rbc has a few predefined variables that can't be
set in the product/board config makefiles, unless they're
set to their predefined values. Exclude these variables
from the board config input variables so that they don't
conflict.

Bug: 201700692
Test: m RBC_BOARD_CONFIG=1 mainline_system_x86_64-userdebug (ninja files differ still but this fixes a compilation error)
Change-Id: Idc11b2c5029d28116236b289ad1f09afaaf83cc3