DOWNLOAD_WGET: use -O instead of -P to set output file

Some URLs use url-encoded arguments to specify the file to download.
Wget will then use an output file name which includes all the ampersands
and such.  However, this is not what we see from buildroot.

E.g.:
FOO_SITE="http://git.foo.com/?p=foo.git;a=blob;f=

"
FOO_SOURCE="foo.tgz"

wget will download this into $(DL_DIR)/index.html?p=foo.git;a=blob;f=foo.tgz
buildroot thinks it's in $(DL_DIR)/foo.tgz

To make sure the view of wget and buildroot are consistent, specify
the output file explicitly.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>