Moved udhcpc.user file to installer script

[ilario]

Fix #927
Taken inspiration from these other uci-defaults files:
https://github.com/libremesh/lime-packages/blob/2c4b783f68860f2542b3910fc6c46f9566b1b205/packages/lime-proto-bmx6/files/etc/uci-defaults/85-add-bmx6-addresses-to-hosts
and
https://github.com/libremesh/lime-packages/blob/f4f8d004ff8564452ab705b02687eff9cc063334/packages/lime-app/files/etc/uci-defaults/95-lime-app-rpc-acl
Should work, but I still have to test it on a router.

[codecov-commenter]

Codecov Report

Merging #928 (18c2ff3) into master (5377f5d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #928   +/-   ##
=======================================
  Coverage   76.73%   76.73%           
=======================================
  Files          48       48           
  Lines        4074     4074           
=======================================
  Hits         3126     3126           
  Misses        948      948           

Impacted Files	Coverage Δ
packages/pirania/tests/pirania_test_utils.lua	100.00% <0.00%> (ø)

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[ilario]

To me it does not look like there is any need to update the tests, is this correct @spiccinini?

[G10h4ck]

@ilario see my coments at #927

[ilario]

Just tested this PR and it is broken.
@G10h4ck pointed out that we could use the /etc/udhcpc.user.d/ directory and place stuff in there. This way is much better so I am going to close this PR.
The doubt is whether that directory will work also on OpenWrt 19.07. To be tested.