main/mkinitfs: bump pkgrel

busybox 1.23.0 before 1.23.0-r2 had a broken modprobe that caused disk
controllers not be loaded from initramfs. This was fixed with
busybox-1.23.0-r2 but we need regenerate initramfs images that was
created with a bad modprobe so we bump pkgrel for mkinitfs to force
regeneration of initramfs.