ArmVirtPkg: build the TFTP command into the UEFI shell

ArmVirtPkg: 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 FVMAIN size only by 8192 bytes, in my AARCH64 build of
VirtArmQemu.dsc, and the final size increase (after LZMA compression)
that is visible in the FVMAIN_COMPACT volume is merely 1976 bytes.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18555 6f19259b-4bc3-4df7-8a09-765794883524