[nrf toup] cmake: check BOARD before setting default

Wil go upstream when multi image is added upstream.

Currently the BOARD variable is not checked before setting the
default value. This causes issues in a multi image context.

Fix this by checking if the BOARD variable is set before
setting it to its default value.

Signed-off-by: Håkon Øye Amundsen <[email protected]>

Author: hakonfam

boot/zephyr/CMakeLists.txt

@@ -8,7 +8,11 @@ cmake_minimum_required(VERSION 3.8.2)
 
 # Board-specific CONF_FILES should get merged into the build as well.
 # Default to qemu_x86 if no board has been specified.
-set(BOARD qemu_x86)
+# If BOARD is already defined, mcuboot is being built in a multi image
+# context, and should default to its parent image BOARD.
+if (NOT BOARD)
+  set(BOARD qemu_x86)
+endif()
 
 # Add a common dts overlay necessary to ensure mcuboot is linked into,
 # and fits inside, the boot partition. (If the user specified a