#### Why I did it
Failed to build sonic-dhcp6relay_1.0.0-0_amd64.deb
#### How I did it
src/dhcprelay has git submodule.
Dependency files by "git ls-files" are not picked files in submodules.
Add --recurse-submodules, work again.
#### How to verify it
make all
Why I did it
src/dhcprelay is being split out to be its own submodule.
How I did it
Add existing dhcprelay commits into the new repo.
Clean up Makefile (sonic-net/sonic-dhcp-relay@772625f)
Add LGTM config (sonic-net/sonic-dhcp-relay@5cc0889)
Add Azure pipeline config (sonic-net/sonic-dhcp-relay@c79cdb7)
Add submodule reference, renaming most references of dhcp6relay to dhcprelay (to reflect that this will not just be for IPv6 in the future).
How to verify it
Successful run of LGTM is tested at sonic-net/sonic-dhcp-relay#4. Failure run of LGTM is tested at sonic-net/sonic-dhcp-relay#3.
Azure pipeline is run for each commit/PR, and will build for amd64, armhf, and arm64. UT/code coverage check is not yet done.
Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>
