uboot_env.c: add flagstype field to yaml

[liuming50]

Currently For a uboot env device like usb or mmc, the flagstype is explicitly set to FLAGS_INCREMENTAL, there is no way to choose FLAGS_BOOLEAN for them, which seems is not right.

Fix that by adding a flagstype field in yaml file.

Reported-by: Mathias Thore [email protected]