pkg-download.mk: support detection of URI schemes in DOWNLOAD

When using one of the package infrastructures, the DOWNLOAD function
uses $(PKG)_SITE_METHOD to determine the appropriate download method,
which is autodetected based on the URI if none was explicitly set.

When the DOWNLOAD function is called directly, for example when
downloading a toolchain, or from a package that does not use one of
the package infrastructures, the SITE_METHOD is not autodetected,
and thus the download defaults to wget.

This patch adds URI scheme detection directly to the DOWNLOAD method,
in case no SITE_METHOD was detected yet.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>