syslinux: deduplicate common files

from the commit log:

there are several files of which identical duplicate copies are held in:

• share/bootloaders/extlinux
• share/bootloaders/pxelinux
• share/bootloaders/isolinux
• share/bootloaders/syslinux

it is a pain to maintain this from a development standpoint, having to copy modified config files into the other directories each time changes are made and mistakes have been made before due to this.

this creates a new folder share/bootloaders/syslinux_common and moves them to this new directory.

it also expands the binary_syslinux stage to use it, with it now constructing the installed set of bootloader files as follows:

1. copy {LB_DIR}/bootloaders/syslinux_common/*
2. copy {LB_DIR}/bootloaders/{syslinux|isolinux|extlinux|pxelinux}/* on top
3. copy config/bootloaders/syslinux_common/* on top
4. copy config/bootloaders/{syslinux|isolinux|extlinux|pxelinux}/* on top

note, to explain part of the binary_syslinux change, instead of just copying the correct bootloader folder full of the files, we now make the target bootloader specific directory, then copy the contents of source directories into it.

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side notes:

1. i noticed a splash replacement bug from a previous commit, which is fixed by MR #186 which applies on top of this
2. the condition for installing librsvg2-bin is ugly, but happens to be tidied up by MR #186