OvmfPkg: Allow HTTP connections if HTTP Boot enabled

OvmfPkg: Allow HTTP connections if HTTP Boot enabled

v2
* Move the setting above the "!ifndef $(USE_OLD_SHELL)" part.
* Un-indent the setting to column zero.
(Comments from Laszlo)

Overwrite the value of PcdAllowHttpConnections to allow HTTP
connections if HTTP Boot enabled (-D HTTP_BOOT_ENABLE).

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Justen Jordan L <jordan.l.justen@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>