How to install mkimage:

method 1,
    sudo apt-get install uboot-mkimage
or
    sudo apt-get install u-boot-tools:i386

method 2,
    step 1: compile u-boot
    step 2: cp source/uboot/u-boot/tools/mkimage /usr/bin/

How to install mkenvimage:
    step 1: compile u-boot
    step 2: cp source/uboot/u-boot/tools/mkenvimage /usr/bin/