Every time you update your CentOS Linux and the update includes a new kernel image update the system will not remove your old kernel but it will cumulatively add new kernel to the top of your Linux kernel installed list. Normally, this does not present any issue to your running system and you are not required to take any action to remove any old and unused kernel images.

The reason why you may wish to remove/uninstall unused kernel images is that you need to reduce disk usage space of your system, especially if your /boot mount point is mounted separately and has a limited disk space
$ df -h /boot/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       331M  198M  112M  64% /boot

debootstrap is Debian GNU/Linux bootstrapper which allows you to install Debian base systems such as Debian or Ubuntu into a directory of your currently running system. This config is a getting started guide on how to debbootstrap on CentOS/RHEL. The debootstrap package is not available in a standard CentOS repository. To make it available we first need to enable EPEL repository. Download EPEL repository package:
# wget http://dl.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-5.noarch.rpm
and install it using RPM command:
# rpm -Uvh epel-release-7-5.noarch.rpm
The EPEL repository should now be enabled. To confirm run:
# yum repolist | grep epel                                                                                  
 * epel: epel.mirror.digitalpacific.com.au

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