OvmfPkg: build the TFTP command into the UEFI shell

OvmfPkg: build the TFTP command into the UEFI shell

The TFTP command is easy to use, it has very nice documentation
(accessible with "HELP TFTP" in the shell), and it's a very versatile tool
for downloading files from the host to the guest, via virtual network,
while the guest is in the UEFI shell.

Even better, enabling this command in the shell increases the uncompressed
DXEFV size only by 12896 bytes, in my X64 build, and the final size
increase (after LZMA compression) that is visible in the FVMAIN_COMPACT
volume is merely 2576 bytes.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18554 6f19259b-4bc3-4df7-8a09-765794883524