common-*-boot-tpl: Use $PATH to find busybox
Under the Buster-based v2019dev0 busybox is not longer available from /usr/lib/initramfs-tools/bin so let's rely on PATH to find where it actually is instead of hardcoding its location.
Under the Buster-based v2019dev0 busybox is not longer available from /usr/lib/initramfs-tools/bin so let's rely on PATH to find where it actually is instead of hardcoding its location.