Cosmetic fixes to the Makefile

This improves indentation and comment
Restore the syslinux configuration

This links to various parts of the isolinux configuration. Note that,
unlike with isolinux, there is no syslinux.bin for the main syslinux
binary. Instead, this main binary is installed using the "syslinux"
command, which comes from the host.

For this reason, vesamenu.c32 is also linked to the host version and not
the embedded isolinux version, to prevent problems with version
mismatches (at the expense of reintroducing #170 for the hdd version of
the host sylinux version is older than 6).
Ignore build products and side effects
Check that live-build version is not too new

Live-build 4.0 contains incompatible changes
Allow running as non-root again

If running as root, nothing changes. If not running as root, sudo is
inserted only when needed.