uboot-imx

tools/env: lookup dev_type directly from flash_read_buf/flash_write_buf

flash_write_buf already looks up size/offset/#sector from struct
envdev_s. It can look up mtd_type as well. Same applies to
flash_read_buf. Makes the interface simpler

Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>