OvmfPkg: Add tftp dynamic command

OvmfPkg: Add tftp dynamic command

The TFTP command was converted from a NULL class library instance
to a dynamic shell command in commit 0961002352e9.
This patch complements commit f9bc2f876326, which only removed the
old library, but didn't add the new dynamic command。

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@linaro.org>