netifd: read udhcpc user scripts from directory

netifd: read udhcpc user scripts from directory

Placeholder DHCP user scripts were added recently.

These files make package-based installations of such scripts more difficult.
Pull user callbacks from directories instead to allow packages and users to
install co-existing scripts more easily.

References:
130118f7a netifd: add a udhcpc.user placeholder script

Signed-off-by: Leon M. George <leon@georgemail.eu>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]
(cherry picked from commit 467c32600cc575fcb67c5f01ad32e02141220ceb)