base-files: execute package's "postinst" after executing uci-defaults

base-files: execute package's "postinst" after executing uci-defaults

Allow "postinst" scripts to perform extra actions after applying all
kind of fixups implemented using uci-defaults.

This is needed e.g. by uhttpd-mod-ubus which after installation in a
running systems needs to:
1. Update uhttpd config using its uci-defaults script
2. Reload uhttpd

While this approach makes sense there is a risk it'll blow up some
corner case postinst usages. There is only 1 way to find out.

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>