OvmfPkg: Add HttpBoot support

OvmfPkg: Add HttpBoot support

This commit introdues a new build option to OvmfPkg: HTTP_BOOT_ENABLE.
When HttpBoot is enabled, a new Network boot option will show in the
boot manager menu with the device path like this:

PciRoot(0x0)/Pci(0x3,0x0)/MAC(525400123456,0x1)/IPv4(0.0.0.0)/Uri()

It works like the PXE one but fetches the NBP from the given http
url instead of the tftp service.

A simple testing environment can be set up with the QEMU tap network
and dnsmasq + lighttpd.

Here is the example of the dnsmasq config:

  interface=<tap interface>
  dhcp-range=192.168.111.100,192.168.111.120,12h
  dhcp-option=60,"HTTPClient"
  dhcp-boot="http://<tap ip>/<efi file>"

It's similar to the PXE server settings except the tftp function is
disabled, the option 60 must be "HTTPClient", and the boot uri is a
http url.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Ching-Pang Lin <glin@suse.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-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@18258 6f19259b-4bc3-4df7-8a09-765794883524