tools/mkimage: copy mkenvimage to bin directory

tools/mkimage: copy mkenvimage to bin directory

For some boards u-boot binary environment file is required,
which is generated by mkenvimage utility. But in OpenWrt there
is no separate support for mkenvimage, which is a part of u-boot tools.
mkenvimage gets built in u-boot/tools as well as mkimage anyways.

So lets just copy mkenvimage to the $(STAGING_DIR_HOST)/bin/ directory.

Signed-off-by: Evgeniy Didin <[email protected]>
CC: Alexey Brodkin <[email protected]>
CC: Hauke Mehrtens <[email protected]>
CC: John Crispin <[email protected]>