[setup-bootable] Check if syslinux.cfg is correct when doing upgrade
When upgrading from 2.x to 3.x, the setup-bootable script should check if syslinux.cfg is correct and either fix or warn about it.
The folowing command is recommended when upgrading:
setup-bootable -u alpine-3.1.1-x86_64.iso /media/$LBU_MEDIA
but it leaves you in an unbootable state, because the kernel files have been renamed.
(from redmine: issue id 3841, created on 2015-01-30, closed on 2019-06-11)