Allow setting the boot-order on CMake CLI

Change the BOOT_ORDER variable to a cache variable to allow setting
the value from CMake command line using -DBOOT_ORDER=<value>.

Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
Change-Id: I43c1fc68f4123115e8ec7841a133e702837658fd
diff --git a/deployments/se-proxy/config/default-opteesp/CMakeLists.txt b/deployments/se-proxy/config/default-opteesp/CMakeLists.txt
index a9b1bad..a1c2d8f 100644
--- a/deployments/se-proxy/config/default-opteesp/CMakeLists.txt
+++ b/deployments/se-proxy/config/default-opteesp/CMakeLists.txt
@@ -25,7 +25,7 @@
 set(SP_FFA_UUID_CANON "${TS_RPC_UUID_CANON}")
 
 set(SP_HEAP_SIZE "32 * 1024" CACHE STRING "SP heap size in bytes")
-set(SP_BOOT_ORDER "2")
+set(SP_BOOT_ORDER "2" CACHE STRING "Boot order of the SP")
 set(TRACE_PREFIX "SEPROXY" CACHE STRING "Trace prefix")
 
 target_include_directories(se-proxy PRIVATE