oe-selftest: fitimage improve bb_vars access

oe-selftest: fitimage improve bb_vars access

Make the code slightly more robust by using e.g.
bb_vars.get('UBOOT_SIGN_ENABLE') instead of bb_vars['UBOOT_SIGN_ENABLE']
for variables which are potentially undefined.
This is a general cleanup but also a preparation for additional test
cases.
Log bb_vars in verbose mode.
Drop one no longer used log message.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>