dhcpcd: move libexecdir

The default is $PREFIX/libexec which ends up in an ugly and non-standard
/libexec dir in the target.
Move it to $PREFIX/lib/dhcpcd which is far more nicer and suitable.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>