linux-headers: prepare to support custom tarball/git

linux-headers: prepare to support custom tarball/git

This commit reworks how linux-headers.mk is written to prepare adding
support for custom tarball/git fetching for the Linux kernel headers.

Basically, the idea is to have a single condition at the beginning of
the file that defines a number of LINUX_HADERS_* variables depending
on whether "kernel headers same as kernel" is used or not, and then
use these variables in the rest of the .mk file.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
[Thomas: split from the original patch "package/linux-headers: add
support for custom headers" from Mark. The commit log is entirely
mine.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>