Firewall-friendly download methods for contrib src

After this commit "make prepare" uses HTTP, HTTPS, or FTP where possible
fvor downloading third-party source codes. This prevents problems with
strict firewall rules where only selected ports are usable.

Unfortunately, git.l4android.org does not support Git via HTTP and,
therefore, the sources need a working Git port (9418).

Fixes #443.

base-codezero/Makefile

@@ -6,7 +6,7 @@
 
 VERBOSE    ?= @
 ECHO        = @echo
-GIT_URL     = git://git.l4dev.org/codezero.git
+GIT_URL     = http://git.l4dev.org/codezero.git
 GIT_REV     = 6fa4884a5a1cf6207372f69ae01e5faa6d5a39c8
 CONTRIB_DIR = contrib
 PATCHES     = $(shell find patches -name *.patch)

base-nova/Makefile

@@ -7,7 +7,7 @@
 
 VERBOSE     ?= @
 ECHO        = @echo
-GIT_URL     = git://github.com/IntelLabs/NOVA.git 
+GIT_URL     = https://github.com/IntelLabs/NOVA.git 
 GIT_REV     = ad92f48564b1ed53e67f58e194c918af4da6a32d
 CONTRIB_DIR = contrib
 PATCHES     = $(shell find patches -name '*.patch')

dde_ipxe/Makefile

@@ -7,7 +7,7 @@
 
 VERBOSE     ?= @
 ECHO         = @echo
-GIT_URL      = git://git.ipxe.org/ipxe.git
+GIT_URL      = http://git.ipxe.org/ipxe.git
 GIT_REV      = 34a81da0c8dd47628a7408cd3f8d8a926d1ecbee
 CONTRIB_DIR  = contrib
 PATCH_FILE   = patches/dde_ipxe.patch

ports/ports/vancouver.mk

@@ -1,6 +1,6 @@
 VANCOUVER_REV     = a7f4c2de42247e7a7c6ddb27a48f8a7d93d469ba
 VANCOUVER         = vancouver-git
-VANCOUVER_URL     = git://github.com/TUD-OS/NUL.git
+VANCOUVER_URL     = https://github.com/TUD-OS/NUL.git
 
 #
 # Check for tools