cmd: nvedit: Add sub-command 'env info'

Add sub-command 'env info' to display environment information:
- env_valid : is environment valid
- env_ready : is environment imported into hash table
- env_use_default : is default environment using

This command can be optionally used for evaluation in scripts:
[-d] : evaluate whether default environment is used
[-p] : evaluate whether environment can be persisted
The result of multiple evaluations will be combined with AND.

Signed-off-by: Leo Ruan <tingquan.ruan@cn.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Do not enable by default]
Signed-off-by: Tom Rini <trini@konsulko.com>