OvmfPkg: Don't include TftpDynamicCommand in XCODE5 tool chain

OvmfPkg: Don't include TftpDynamicCommand in XCODE5 tool chain

https://bugzilla.tianocore.org/show_bug.cgi?id=1355
XCODE doesn't support HII resource section. TftpDynamicCommand driver depends
on HII resource section. To let OvmfPkg boot to shell on XCODE5 tool chain,
don't include TftpDynamicCommand driver.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>