make.py: Add option for skipping firmware build
make.py: Add option for skipping firmware build

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>

mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware