boot/opensbi: add options to enable/disable image installation

Until now, whenever a BR2_TARGET_OPENSBI_PLAT value was specified,
opensbi.mk was assuming that both fw_jump and fw_dynamic would be
produced. However, this is not the case: the OpenSBI per-platform
config.mk can decide which image to build.

As an example, the config.mk for VIC7100-based BeagleV only enables
producing the fw_payload image.

This commit adds three options to enable the installation of images:
one for fw_jump, one for fw_dynamic, one for fw_payload.

The options for fw_jump and fw_dynamic are "default y" when
BR2_TARGET_OPENSBI_PLAT is not empty, to preserve existing behavior.

The option for fw_payload is forcefully selected when either Linux or
U-Boot are selected as payloads.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>