Split the code that merge the system uEnv to new function. While we're here,
clean up the logic to e.g. use `ot_openat_ignore_enoent()`.
Closes: #1138
Approved by: cgwalters
When using the
OSTREE_SYSROOT_SIMPLE_WRITE_DEPLOYMENT_FLAGS_NOT_DEFAULT flag, the
deployment is said to be added after the booted or merge deployment.
Fix the condition to do so instead of adding it in the second place.
Closes: #1097
Approved by: cgwalters
Guy Shapiro