makes start.S, entry.S and link.lds configurable

CMakeLists.txt

@@ -1,6 +1,11 @@
 cmake_minimum_required(VERSION 3.21)
 project(mnrs-bsp LANGUAGES ASM C)
-
+set(LINKER_SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/env/${BOARD}/link.lds"
+    CACHE FILEPATH "Linker script to use for BSP linking")
+set(BSP_STARTUP "${CMAKE_CURRENT_SOURCE_DIR}/env/start.S"
+    CACHE FILEPATH "Path to the BSP startup assembly file")
+set(BSP_TRAP_HANDLER "${CMAKE_CURRENT_SOURCE_DIR}/env/entry.S"
+    CACHE FILEPATH "Assembly file implementing trap handler")
 if(NOT DEFINED BOARD)
 message(FATAL_ERROR "No Board selected")
 endif()
@@ -17,7 +22,7 @@ if(SEMIHOSTING)
     add_compile_definitions(SEMIHOSTING)
 endif()
 
-add_library(startup STATIC env/start.S env/entry.S)
+add_library(startup STATIC ${BSP_STARTUP} ${BSP_TRAP_HANDLER})
 target_include_directories(startup PUBLIC env include)
 
 add_subdirectory(libwrap)
@@ -25,7 +30,7 @@ add_subdirectory(libwrap)
 add_library(bsp STATIC env/${BOARD}/init.c)
 target_link_libraries(bsp PUBLIC startup wrap)
 target_include_directories(bsp PUBLIC env/${BOARD})
-target_link_options(bsp INTERFACE LINKER:--no-warn-rwx-segments -nostartfiles -T ${CMAKE_CURRENT_SOURCE_DIR}/env/${BOARD}/link.lds)
+target_link_options(bsp INTERFACE LINKER:--no-warn-rwx-segments -nostartfiles -T ${LINKER_SCRIPT})
 
 if(SEMIHOSTING)
     target_include_directories(bsp INTERFACE include)