env: unify logic to check and apply changes

env: unify logic to check and apply changes

The logic of checking special parameters (e.g. baudrate, stdin, stdout,
for a valid value and/or whether can be overwritten) and applying the
new value to the running system is now all within a single function
env_check_apply() which can be called whenever changes are made
to the environment, no matter if by set, default or import.

With this patch env_check_apply() is only called by "env set",
retaining previous behavior.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>