base-files: rc.common: fix missing EXTRA_HELP texts

base-files: rc.common: fix missing EXTRA_HELP texts

Commit ed5b9129d7a4 ("base-files: implement generic service_running")
has added EXTRA_HELP variable, thus overriding already available
EXTRA_HELP text available in other init scripts, resulting in the
missing help text from services like dropbear for example.

So fix this regression by appending EXTRA_HELP text provided by the
other init scripts into the one provided by the script itself.

Fixes: ed5b9129d7a4 ("base-files: implement generic service_running")
Signed-off-by: Peter Stadler <peter.stadler@student.uibk.ac.at>
[commit title/description facelift, fixes tag, fixed From:, pkg bump]
Signed-off-by: Petr Štetiar <ynezz@true.cz>