Custom DTS overlay file support

Add KernelCustomDTSOverlay option to append user provided DTS overlay
files. This way users do not need to modify the seL4 kernel sources
just because they use a hardware module differently. Also useful for
defining VM memory regions.

Signed-off-by: Indan Zupancic <Indan.Zupancic@mep-info.com>

config.cmake

@@ -108,6 +108,18 @@ if(DEFINED KernelDTSList AND (NOT "${KernelDTSList}" STREQUAL ""))
     )
     set(config_file "${CMAKE_CURRENT_SOURCE_DIR}/tools/hardware.yml")
     set(config_schema "${CMAKE_CURRENT_SOURCE_DIR}/tools/hardware_schema.yml")
+    set(
+        KernelCustomDTSOverlay ""
+        CACHE FILEPATH "Provide an additional overlay to append to the selected KernelPlatform's \
+        device tree during build time"
+    )
+    if(NOT "${KernelCustomDTSOverlay}" STREQUAL "")
+        if(NOT EXISTS ${KernelCustomDTSOverlay})
+            message(FATAL_ERROR "Can't open external overlay file '${KernelCustomDTSOverlay}'!")
+        endif()
+        list(APPEND KernelDTSList "${KernelCustomDTSOverlay}")
+        message(STATUS "Using ${KernelCustomDTSOverlay} overlay")
+    endif()
 
     find_program(DTC_TOOL dtc)
     if("${DTC_TOOL}" STREQUAL "DTC_TOOL-NOTFOUND")