configs: tegra-common-post: make PXE and DHCP boot targets optional

Disabling the network related features in defconfig causes build to
fail so make them optional.

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS Grouper E1565
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # LG P895 T30
Signed-off-by: Jonas Schwöbel <jonasschwoebel@yahoo.de>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>