syslinux: Allow mtools to be optional

syslinux: Allow mtools to be optional

Adds a PACKAGECONFIG to syslinux to determine if the mtools version of
the utilities is installed or not. The difference between the two
versions is that the mtools version can be used by any user with write
permission, while the non-mtools can only be used by root.

The syslinux-nomtools package is removed, as it was empty and doesn't
appear to be used anyway

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>