package/dhcp: add host-gawk to global dependencies and build environment

package/dhcp: add host-gawk to global dependencies and build environment

DHCP package may silently fail to install binaries to the target image.
The problem occurs when buildroot output/host and build server provide
different flavors of awk. For instance, mawk on build server and gawk
in buildroot output/host. In this case isc-dhcp configure script detects
gawk in output/host and generates Makefiles specifying gawk without
absolute path. During Buildroot installation phase, those Makefiles
are used to install dhcp binaries. They attempt to use gawk without
absolute path. However build host does not have gawk.

To resolve the issue add host-gawk to dependencies and specify absolute
path to host-gawk in dhcp configure script using DHCP_CONF_ENV.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>