uboot: add support for patch files and URLs

The existing u-boot patch option only allowed directories to be
specified. This adds support for URLs using similar code as found
in linux/linux.mk. Local files are also handled now.

This change is useful for Intel Edison support, so that Intel's u-boot
patch can be downloaded rather than stored in the Buildroot source tree.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>