syslinux: convert to GENTARGETS

Moreover, the installation of pxelinux and/or isolinux is now handled
as suboptions, like we traditionally do for other packages. This allows
to have a single option (BR2_TARGET_SYSLINUX) that enables the
syslinux package.

[Peter: drop basename in install step]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>