RISC-V: Rename CONFIG_CMDLINE_OVERRIDE to CONFIG_CMDLINE_FORCE

The device tree code looks for CONFIG_CMDLINE_FORCE, but we were using
CONFIG_CMDLINE_OVERRIDE.  It looks like this was just a hold over from
before our device tree conversion -- in fact, we'd already removed the
support for CONFIG_CMDLINE_OVERRIDE from our arch-specific code so it
didn't even work any more.

Thanks to Mortiz and Trung for finding the original bug, and for Michael
for suggeting a better fix.

CC: Trung Tran <trung.tran@ettus.com>
CC: Michael J Clark <mjc@sifive.com>
Reviewed-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>