allow integration of custom kernels in linbo filesystem

[HappyBasher]

Sometimes a certain hardware only works with a certain kernel version. Therefore, it should be possible to integrate its own kernel versions into the linbo file system.

[HappyBasher]

• update-linbofs: use custom kernel optional.
• add custom kernel config file.
• make-linbo-iso.sh: Increase EFIIMAGESIZE to 256M.

[HappyBasher]

See https://github.com/linuxmuster/linuxmuster-linbo7#integrate-your-own-kernel.

[HappyBasher]

add ability to use Linbo's legacy kernel 5.15.138 alternatively.
Fix real modules path.

[HappyBasher]

Done.