package/linux-headers: add support for fetching from custom tarball/git

package/linux-headers: add support for fetching from custom tarball/git

Add support for building toolchains against custom headers. Allows
the selection of a manual version, custom tarball or custom git
repository for the kernel headers. This enables toolchains to be
built against custom kernel headers without having to build a full
kernel.

This is particularly useful for new architectures, such as RISC-V
where updated kernel headers may not have made it into the mainline
kernel yet.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>