build system: rename mips/bootblock.inc to mips/bootblock.S

This is inspired by the commit listed below, but rewritten to match
upstream, and split in smaller pieces to keep intent clear.

Change-Id: I192fa50989b586fd8e967d4c22db56ac9de7a30e
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Based-On-Change-Id: I50af7dacf616e0f8ff4c43f4acc679089ad7022b
Based-On-Signed-off-by: Julius Werner <jwerner@chromium.org>
Based-On-Reviewed-on: https://chromium-review.googlesource.com/219170
Reviewed-on: http://review.coreboot.org/9108
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>
2 files changed