live-image: disable mkosi Bootable (PXE doesn't need a bootloader)

mkosi was failing with "systemd-boot was not found at
usr/lib/systemd/boot/efi" because Bootable=yes expects systemd-boot
installed *inside* the image for EFI boot. This image is only ever
PXE-booted — iPXE loads vmlinuz+initrd from TFTP directly, so the
rootfs itself needs no bootloader.

Switching to Bootable=no drops the EFI-image assembly step; the
linux-image-amd64 postinst still creates /vmlinuz and /initrd.img
symlinks that the top-level Makefile copies into the bundle.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

live-image/mkosi.conf

@@ -15,7 +15,11 @@ Format=directory
 Output=build
 
 [Content]
-Bootable=yes
+# PXE live image — iPXE loads vmlinuz+initrd from TFTP, so the rootfs
+# itself doesn't need an EFI bootloader. Bootable=no skips mkosi's
+# systemd-boot/bootctl dance; we still get /vmlinuz + /initrd.img
+# symlinks courtesy of the linux-image-amd64 postinst.
+Bootable=no
 BuildPackages=
 Packages=
     systemd