bluez5_utils: define FIRMWARE_DIR for hciattach_bcm43xx

The tool hciattach_bcm43xx defines the default firmware path in `/etc/firmware`,
but the Broadcom firmware blobs are usually stored in `/lib/firmware`.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>