Add generic opt_VARS/opt_VARS_OFF.

OPT1_VARS= foo=bar baz+=bam

will set FOO to bar and append bam to BAZ if OPT1 is enabled. <opt>_VARS_OFF
works the same way, if the option is disabled.

Reviewed by: bapt
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D3410