Usage of device-tree for blobs

[Heinrich Schuchardt]

Hello Simon,

some boards like qemu-riscv64_defconfig do not use any device-tree at
build time. A device-tree is only supplied at runtime by the prior boot
stage (CONFIG_OF_PRIOR_STAGE=y).

In doc/develop/devicetree/intro.rst you suggest to put binary blobs into
the device-tree.

Could you, please, update the documentation to explain how adding blobs
to the device-tree works in the different scenarios depending on the
values of:

CONFIG_OF_EMBED
CONFIG_OF_SEPARATE
CONFIG_OF_BOARD
CONFIG_OF_HOSTFILE
CONFIG_OF_PRIOR_STAGE

It would be especially important to understand how one can develop a
board independent feature which works for all of these settings.

Please, describe CONFIG_OF_PRIOR_STAGE in
doc/develop/devicetree/control.rst.

Best regards

Heinrich