setup-environment: simplify error handling

Make better use of the setup-mel-builddir exit code to determine what actions
we take.

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

This leverages a post-setup-environment snippet to load an additional
local.conf.append snippet, to avoid having to create one for each and every
supported qemu MACHINE.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Merge pull request #368 from kergoth/sb-3858

Implement and use local.conf.append support