ArmVirtPkg/ArmVirtQemu: port HTTP_BOOT_ENABLE from OvmfPkg

ArmVirtPkg/ArmVirtQemu: port HTTP_BOOT_ENABLE from OvmfPkg

* From commit ab44a6e833fa ("OvmfPkg: Add HttpBoot support", 2015-08-23):
  - introduce HTTP_BOOT_ENABLE build define
  - resolve HttpLib class
  - include NetworkPkg drivers DnsDxe, HttpDxe, HttpBootDxe

* From commit 1a85139d9eac ("OvmfPkg: Build HTTP utilities driver",
  2015-08-28):
  - include NetworkPkg driver HttpUtilitiesDxe

* From commit 4b2fb7986d57 ("OvmfPkg: Allow HTTP connections if HTTP Boot
  enabled", 2017-01-19):
  - set PcdAllowHttpConnections to TRUE

> ## Indicates whether HTTP connections (i.e., unsecured) are permitted or
> ## not.
> # TRUE  - HTTP connections are allowed. Both the "https://" and
> #         "http://" URI schemes are permitted.
> # FALSE - HTTP connections are denied. Only the "https://" URI scheme is
> #         permitted.
> # @Prompt Indicates whether HTTP connections are permitted or not.
> gEfiNetworkPkgTokenSpaceGuid.PcdAllowHttpConnections|FALSE|BOOLEAN|...

TLS_ENABLE (for HTTPS) is not ported for the time being.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>