mkconfig: remove bashisms and cleanup config.mk generation

This adds some cleanup to mkconfig related to SPL support.  Bash
specific script has been replaced with awk for better shell
compatibility.  config.mk generation is done through a subshell and
single redirect to improve readability.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>