setup-environment: add local.conf snippet support

For each configured layer, check for conf/local.conf.append, and if it exists,
append it to local.conf, in a bracketed block. It will automatically remove an
existing block for this file if it exists, to handle reconfiguration of
existing build directories. This should be much more convenient than using
post-setup-environment for the most common case.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
setup-environment: also parse local.conf.append.$MACHINE

This will allow for machine specific adjustments and fixups, which is
particularly useful when dealing with bsp layers outside your control.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
setup-environment: use local.conf.append.minnow

Signed-off-by: Christopher Larson <chris_larson@mentor.com>