package/rcw: add option to generate the RCW

package/rcw: add option to generate the RCW

When the rcw tools is built, it is probably to generate the RCW (Reset
Configuration Word) of the target. So it makes sense to do this directly
in the Buildroot infra and install it in the images directory, just like
we do for e.g. device tree blobs.

Add an option BR2_PACKAGE_HOST_RCW_CUSTOM_PATH that allows to specify
the source for the RCW. And while we're at it, make it possible to
specify multiple .rcw and .rcwi files, just like for device trees.

Signed-off-by: Jared Bents <jared.bents@rockwellcollins.com>
[Arnout:
  * Reword help text and commit message
  * Remove definition of RCW_DELIVERY_FILE
  * Define and use RCW_FILES
  * Enclose everything in ifneq($(RCW_FILES),)
  * Enclose error in ifeq($(BR_BUILDING),y)
  * Add actual error message
  * Don't limit to only %.rcwi files
  * Don't delete *.bin
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>