uboot-envtools: update to 2018.03

Several changes has been made:

+ AES support has been removed by
  upstream commit c6831c7 (2017-11-14)
  [remove patch "200-fw_env_no_aes.patch"]

+ Support for UBI volumes has beed added by
  upstream commit 34255b9 (2017-11-15)
  [remove patch "300-support-env-in-ubivol-chardev.patch"]

+ A command line argument has beed added ("-c") to manually indicate
  the location of the environment configuration file

Also, patch "400-u-boot-2015.10-stdint.patch" is no longer
necessary, and the config option to enable UBI support has
been removed.

Size comparisons:

fw_printenv size:
Target    Before         After
ar71xx    15,189 bytes   18,133 bytes (+2,944 bytes)
ipq40xx   20,873 bytes   20,987 bytes (+114 bytes)
mvebu     20,881 bytes   20,991 bytes (+110 bytes)
ramips    15,128 bytes   18,072 bytes (+2,944 bytes)

OPKG package size:
Target    Before         After
ar71xx    11,309 bytes   12,875 bytes (+1,566 bytes)
ipq40xx   11,772 bytes   13,299 bytes (+1,527 bytes)
mvebu     11,609 bytes   13,114 bytes (+1,505 bytes)
ramips    10,975 bytes   12,503 bytes (+1,528 bytes)

Compile tested: ipq40xx (musl, glibc, gcc5-musl), ar71xx, mvebu, ramips
Run tested: ipq40xx (ASUS RT-AC58U)

Signed-off-by: Luis Araneda <luaraneda@gmail.com>