mkimage: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/mkimage - from package: uboot-tools

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mkimage - from package: u-boot-tools

You may install the required package with command apt-get install {package}

Error in other languages:
mkimage: bevel nie gevind nie
mkimage: comandă negăsită
mkimage: comando não encontrado
mkimage: commande introuvable
mkimage: command not found
mkimage: käsku ei ole
mkimage: không tìm thấy lệnh
mkimage: komanda nerasta
mkimage: Kommando nicht gefunden.
mkimage: kommandot finns inte
mkimage: komut yok
mkimage: nie znaleziono polecenia
mkimage: níor aimsíodh an t-ordú
mkimage: no se encontró la orden
mkimage: no s'ha trobat l'ordre
mkimage: opdracht niet gevonden
mkimage: parancs nem található
mkimage: perintah tidak ditemukan
mkimage: príkaz nenájdený
mkimage: příkaz nenalezen
mkimage: команда не найдена
mkimage: командата не е открита
mkimage: 命令找不到
mkimage: コマンドが見つかりません