build: allow openwrt.git packages to be replaced by feeds

Currently, replacing a package available in openwrt.git requires
modifications in openwrt.git, or requires duplicating the package in a
feed but with a different name, which causes all kind of problems
related to dependencies (all packages selecting it would have to be
modified accordingly to select the new package).

With this change, if a package with the same name is present both in
feeds/ and package/ folders, the one in feeds/ can override the one
in package/, both in the menuconfig and during the build, by passing the
"-f" option to "./scripts/feeds install"

This mechanism is particularly useful for vendor tree, or in general for
application which needs to replace one particular package which exists
within openwrt.git by a custom/newer version.

Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>

SVN-Revision: 44334