package/dhcp: fix installation with BR2_STATIC_LIBS=y

package/dhcp: fix installation with BR2_STATIC_LIBS=y

Use the "install-sbinPROGRAMS" make target to install the utilities
instead of guessing their locations. We disable libtool for static
builds so the binaries are not always in a ".libs" subdir.

Fixes:
  http://autobuild.buildroot.net/results/0c43c02b8d418b60b4734187abde1da585e9e0ca/
  http://autobuild.buildroot.net/results/e61b246df60af1e59b8f643510557ef2270f2d97/

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>