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 <Evgeniy.Didin@synopsys.com>
CC: Alexey Brodkin <abrodkin@synopsys.com>
CC: Hauke Mehrtens <hauke@hauke-m.de>
CC: John Crispin <john@phrozen.org>