Add bootloader configs to archiso profile (syslinux/efiboot/grub)

The profile declared syslinux + systemd-boot bootmodes but lacked the
required config directories, so mkarchiso would fail. Added from the
official releng profile, rebranded to Bread OS; %PLACEHOLDER% tokens are
substituted by mkarchiso at build time.

iso/syslinux/archiso_tail.cfg

@@ -0,0 +1,35 @@
+LABEL existing
+TEXT HELP
+Boot an existing operating system.
+Press TAB to edit the disk and partition number to boot.
+ENDTEXT
+MENU LABEL Boot existing OS
+COM32 chain.c32
+APPEND hd0 0
+
+# https://www.memtest.org/
+LABEL memtest
+MENU LABEL Run Memtest86+ (RAM test)
+LINUX /boot/memtest86+/memtest
+
+# https://wiki.syslinux.org/wiki/index.php/Hdt_(Hardware_Detection_Tool)
+LABEL hdt
+MENU LABEL Hardware Information (HDT)
+COM32 hdt.c32
+APPEND modules_alias=hdt/modalias.gz pciids=hdt/pciids.gz
+
+LABEL reboot
+TEXT HELP
+Reboot computer.
+The computer's firmware must support APM.
+ENDTEXT
+MENU LABEL Reboot
+COM32 reboot.c32
+
+LABEL poweroff
+TEXT HELP
+Power off computer.
+The computer's firmware must support APM.
+ENDTEXT
+MENU LABEL Power Off
+COM32 poweroff.c32