boot/optee-os: support alternate image files

boot/optee-os: support alternate image files

Some platform may generate specific boot image files instead of
the generic files tee.bin and tee-*_v2.bin when building OP-TEE OS
package.

This change introduces optee-os configuration directive
BR2_TARGET_OPTEE_OS_CORE_IMAGES that allows board configuration
to specify its expected boot image file names.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[Thomas: use the current hardcoded values as the default for the new
config option, to avoid breaking existing setups, and therefore use
$(wildcard ...) to support wildcards]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>