toolchain.inc: Update comments

This fixes some nits that were pointed out in a previous review, and
adds a couple additional comments to explain what is happening.

Change-Id: I1ca4bf59ba79744f79fbe73f4e226feeea1cc2ab
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13019
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed