defaults: fix broken install wrt. bootloaders

a default amd64|i386 iso build (or one which does not override the default bootloaders) fails within the binary_iso stage.

after installing required packages, the following output occurs:

xorriso 1.5.0 : RockRidge filesystem manipulator, libburnia project.

Drive current: -outdev 'stdio:live-image-amd64.hybrid.iso'
Media current: stdio file, overwriteable
Media status : is blank
Media summary: 0 sessions, 0 data blocks, 0 data, 31.0g free
xorriso : NOTE : -as mkisofs: Ignored option '-cache-inodes'
xorriso : WARNING : -volid text problematic as automatic mount point name
xorriso : WARNING : -volid text is too long for Joliet (28 > 16)
xorriso : WARNING : -volid text does not comply to ISO 9660 / ECMA 119 rules
Added to ISO image: directory '/'='/binary'
xorriso : UPDATE :      39 files added in 1 seconds
xorriso : UPDATE :      39 files added in 1 seconds
xorriso : NOTE : Copying to System Area: 432 bytes from file '/usr/lib/ISOLINUX/isohdpfx.bin'
libisofs: NOTE : Aligned image size to cylinder size by 161 blocks
libisofs: MISHAP : Isolinux image too small. We won't patch it.
xorriso : UPDATE :  29.07% done
ISO image produced: 115200 sectors
Written to medium : 115200 sectors at LBA 0
Writing to 'stdio:live-image-amd64.hybrid.iso' completed successfully.

xorriso : NOTE : -return_with SORRY 32 triggered by problem severity MISHAP
E: An unexpected failure occurred, exiting...
P: Begin unmounting filesystems...
P: Saving caches...
Reading package lists... Done
Building dependency tree
Reading state information... Done

this occurs with the default selected bootloaders (syslinux and grub-efi), and clearly relates to syslinux. if you change the bootloaders to "grub-pc"|"grub-pc,grub-efi" then the build succeeds. similarly if you use syslinux but switch to an HDD build then it succeeds.

i do not recall the specifics of syslinux currently to know whether it should be expected to fail for ISO usage, but the obvious simple solution is to avoid its use by switching the default to grub-pc, except for hdd|netboot.

if syslinux is supposed to work for ISO builds, then why it fails needs investigating, and once fixed the default can be switched back. if it is not to be expected to work then there are two questions - 1) why was it ever set as the default (though attributing blame is unhelpful), and more importantly 2) why was there no check in place to catch the incompatible bootloader + image-type combination???

Gbp-Dch: Short