DOWNLOAD: change $1=DIRECTORY_URL, $2=FILE_NAME to $1=FULL_FILE_URL, $2=FILE_NAME

This modifies the definition of DOWNLOAD to receive two arguments:
the first one is the full URL of the file to download, whereas the second
(and optional) is the name the file will have once downloaded.

Same thing with the SOURCE_CHECK_WGET and SCP functions.

All calls to these functions have been changed to the shortest form of
the new API, except for toolchains acquisition. Since there is quite a
number of different toolchains this call to DOWNLOAD is better set to the
generic one.

Signed-off-by: Alvaro G. M <alvaro.gamez@hazent.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Stephan Hoffmann <sho@relinux.de>
Downloading Microblaze LE toolchain works on a clean install
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>