Execute --uboot-cmd even if there are no files to boot

This is useful in certain situations, for example when we want to
execute custom U-Boot command end exit novaboot without waiting for next
U-Boot prompt.

We also wrap kernel loading in an if block to suppress Perl warnings
about undefined $kbin.
Always finish novaboot output with new line character

Previously, printed the character only when --exiton was in effect. It
is necessary to print it even in other cases, for example when novaboot
is started with --no-interaction.

The previous print command was erroneously changed to STDERR, so we
change it back to STDOUT.