uboot-envtools: move mediatek to mediatek_mt7622

All mediatek boards having fw_env accessible through uboot-envtools
belong to be mt7622 subtarget. Move the file, as subtarget-specific
files are supported for a while now.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

diff --git a/package/boot/uboot-envtools/files/mediatek b/package/boot/uboot-envtools/files/mediatek_mt7622
similarity index 100%
rename from package/boot/uboot-envtools/files/mediatek
rename to package/boot/uboot-envtools/files/mediatek_mt7622