scripts: Support for downloading sources directly from git, svn or using...

... local file.

PKG_URL="http://www.somelink.com"
PKG_URL="git,git://git.ffmpeg.org/rtmpdump"
PKG_URL="git,mybranch,git://git.ffmpeg.org/rtmpdump"
PKG_URL="svn,svn://coreboot.org/flashrom/trunk"
PKG_URL="file,~/tmp/test_file-7340f6d.tar.xz"

Comments welcome.