external-toolchain-csl: allow linux-libc-headers

external-toolchain-csl: allow linux-libc-headers

If PREFERRED_PROVIDER_linux-libc-headers is defined as anything
other than external-toolchain-csl then don't PROVIDE, install or
stage the linux headers packaged with the CSL toolchain.

(Updated by Tom Rini to not modify local.conf.sample yet and to use
cp + rm -rf rather than rsync)

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Tested-by: Andrea Galbusera <gizero@gmail.com>
Signed-off-by: Tom Rini <tom_rini@mentor.com>