seabios: fix failure to find python

The failure has been reported by the automatic builders:

http://package19.nyi.freebsd.org/data/103i386-default-build-as-user/412304/logs/errors/seabios-1.9.1.log

This is because SeaBIOS doesn't have a configure phase and the paths of the
required build tools need to be passed to the make command when building.

Approved by: bapt
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D5804