[live-build] "E: Unable to find a source package for syslinux,grub-efi"

[jnqnfe@gmail.com]

another todo list item of mine that I have not gotten around to yet,
that perhaps someone could find time to assist with is that I've
encountered the following error with generation of the source disc:

```
E: Unable to find a source package for syslinux,grub-efi
dpkg-query: no packages found matching syslinux,grub-efi
```

note that I'm using git master and various local branches built on
that; I do not yet know whether or not this occurs with the latest
released version (haven't found time to check yet).

obviously it seems to be treating "syslinux,grub-efi" as a single
package name which is wrong. this string originates from
LB_BOOTLOADERS.

the code that should be handling this in source_debian looks to be the
following:
```
echo "${LB_BOOTLOADERS}" | \
while IFS="," read -r BOOTLOADER
do
	echo "${BOOTLOADER}" >> source-selection.txt
done
```

which is correctly specifying a comma as the separator, so if this is
where the problem originates, I don't know why...