buildroot

configs/arm_juno: add the bootloader build

Thanks to the recent addition of the vexpress-firmware and
arm-trusted-firmware packages, this commit improves the ARM Juno
defconfig to also build a bootloader.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>