reexec: allow overriding boot directory via env

# kexec for dummies

BOOT_DIR="${BOOT_DIR:-/boot}"

(( UID > 0 )) && { echo "You must be root" >&2; exit 1; }

whence -p kexec >/dev/null || { echo "kexec is not in PATH" >&2; exit 1; }

[[ -r /boot ]] || { echo "Unable to read /boot" >&2; exit 1; }

[[ -r "$BOOT_DIR" ]] || { echo "Unable to read $BOOT_DIR" >&2; exit 1; }

read machineid < /etc/machine-id

kernels=( /boot/vmlinuz-*(Nom) /boot/"$machineid"/*/linux(Nom) )

kernels=( "$BOOT_DIR"/vmlinuz*(Nom) "$BOOT_DIR/$machineid"/*/linux(Nom) )

(( ${#kernels[@]} > 0 )) || { echo "No kernel found." >&2; exit 2; }

select kernel in "${kernels[@]}"; do