dhcp: Bump to 4.1.1-P1 and convert to autotools

dhcp 3.0.5 was causing build issues due to its build system not based
on the autotools. Instead of work-arounding dhcp 3.0.5 issues, this
commit bumps to 4.1.1 which uses the autotools as the build
system. The package is therefore converted to the autotools
infrastructure.

The dhcp_xecute patch is no longer needed, this feature seems to have
been merged into the upstream dhcp (according to the manpage).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>