Fix linker script incorrectly selected for bare-metal profile

mbed-os and mbed-baremetal library targets both are created, so we cannot
use if(TARGET mbed-os) or if(TARGET mbed-baremetal) to determine which one
the application target links to. Instead, determine by checking application
target's LINK_LIBRARIES property.

tools/cmake/mbed_target_functions.cmake

@@ -120,18 +120,21 @@ function(mbed_set_post_build target)
     # add linker script. Skip for greentea test code, there the linker script is set in mbed_setup_linker_script()
     if (NOT MBED_IS_STANDALONE)
         if("${ARGN}" STREQUAL "")
-            if(TARGET mbed-os)
+            get_target_property(POST_BUILD_TARGET_LINK_LIBRARIES ${target} LINK_LIBRARIES)
+            if("mbed-os" IN_LIST POST_BUILD_TARGET_LINK_LIBRARIES)
                 get_target_property(LINKER_SCRIPT_PATH mbed-os LINKER_SCRIPT_PATH)
                 target_link_options(${target}
                 PRIVATE
                     "-T" "${LINKER_SCRIPT_PATH}"
             )
-            elseif(TARGET mbed-baremetal)
+            elseif("mbed-baremetal" IN_LIST POST_BUILD_TARGET_LINK_LIBRARIES)
                 get_target_property(LINKER_SCRIPT_PATH mbed-baremetal LINKER_SCRIPT_PATH)
                 target_link_options(${target}
                 PRIVATE
                     "-T" "${LINKER_SCRIPT_PATH}"
             )
+            else()
+                message(FATAL_ERROR "Target ${target} used with mbed_set_post_build() but does not link to mbed-os or mbed-baremetal!")
             endif()
         else()
             message(STATUS "${target} uses custom linker script  ${ARGV1}")