base-files: implement ucidef_set_hostname(), ucidef_set_ntpserver()

Commit 2036ae4 (base-files: support hostname and ntp servers through board.d)
was supposed to implement these procedures but lacked the required changes
to uci-defaults.sh.

Add the missing procedures now to fix config generation on targets relying
on hostname or NTP server presetting.

Fixes FS#754.

Reported-by: Cristian Morales Vega <cristian@samknows.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>