configs/beaglev: enable building of low-level firmware

This commit extends the beaglev_defconfig and its documentation to
build the low-level firmware, and to explain how to reflash it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[yann.morin.1998@free.fr: use typoed-name for beaglev-ddrlnit]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>