ybin --bootonce doesn't work

1) It inverts the logic of a test to see if nvram is available, and
2) It assumes the output of 'nvsetenv VARNAME' will be 'VARNAME=VALUE', rather
than just 'VALUE' (as it is when using the "nvram" utility).

From bugzilla:
  https://bugzilla.redhat.com/show_bug.cgi?id=471425

The attached patch fixes these problems and makes --bootonce work as expected.

Signed-off-by: Roman Rakus <rrakus@redhat.com>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>