The efi-pxeboot test can crash because of an unassigned variable

[Rod Smith]

See:

https://certification.canonical.com/hardware/201706-25573/submission/130689/

Specifically:

Traceback (most recent call last):
  File "/tmp/nest-grcrabbc.ddffe1464fb1472263c562eaa13feeb6fd7958c5aef597cea6e3f81f7455786c/efi-pxeboot", line 149, in <module>
    sys.exit(main())
  File "/tmp/nest-grcrabbc.ddffe1464fb1472263c562eaa13feeb6fd7958c5aef597cea6e3f81f7455786c/efi-pxeboot", line 133, in main
    boot_entries, boot_order, boot_current = discover_data()
  File "/tmp/nest-grcrabbc.ddffe1464fb1472263c562eaa13feeb6fd7958c5aef597cea6e3f81f7455786c/efi-pxeboot", line 78, in discover_data
    return boot_entries, boot_order, boot_current
UnboundLocalError: local variable 'boot_current' referenced before assignment

The exception is tripped on the function's return line. The fix should be easy -- assign all three of the variables the function returns to empty strings before the if/elif/then clauses, to ensure all three have values.