fastboot: use --force to bypass requirements

Allow using --force to bypass flashing requirements
from android-info.txt.

We often provide builds to deviceholders who don't have
an exact matching bootloader or radio image and it is
unimportant.  Having the option to override is useful.

This shouldn't affect bootloader.img so should never
prevent a device from returning to fastboot in case of
gross error.

Test: Downloaded a build w/ different radio.

Got the following output:

...
Checking 'version-baseband'                        FAILED

Device version-baseband is 'xyz'.
Update requires 'abc' or '123'.

requirements not met! but proceeding due to --force
Setting current slot to 'a'                        OKAY [  0.065s]

Bug: 184661990
Change-Id: I8680b8275b854304026cff3b2e663b7ef2594383