do not do stuff for EFI on BIOS

[Batcastle]

There were not checks in the verification script for if the user was
using UEFI or BIOS, before trying to set efi vars. Added those checks
in.

[pep8speaks]

Hello @Batcastle! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file usr/share/system-installer/modules/verify_install.py:

Line 50:47: W605 invalid escape sequence '\E'
Line 50:51: W605 invalid escape sequence '\s'
Line 50:59: W605 invalid escape sequence '\s'
Line 50:80: E501 line too long (80 > 79 characters)
Line 102:80: E501 line too long (87 > 79 characters)
Line 127:80: E501 line too long (86 > 79 characters)