tools/env: complete environment device config early

Currently flash_read completes a crucial part of the environment
device configuration, the device type (mtd_type). This is rather
confusing as flash_io calls flash_read conditionally, and one might
think flash_write, which also makes use of mtd_type, gets called
before flash_read. But since flash_io is always called with O_RDONLY
first, this is not actually the case in reality.

However, it is much cleaner to complete and verify the config early
in parse_config. This also prepares the code for further extension.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Andreas Fenkart