cmake: fixed issue with CONF_FILE pattern when using build type

This commit fixes an issue where specifying a build type config file
in a subfolder named: `prj_<something>` would cause a wrong match and
fail to find Kconfig fragments, as example:

Working: `-DCONF_FILE=myconfigs/prj_debug.conf` would correctly add
myconfigs/boards/<board>_debug.conf to list of conf files.

Failing: `-DCONF_FILE=prj_configs/prj_debug.conf` would fail to locate
prj_configs/boards/<board>_debug.conf, and thus the board specific
config fragment was not added to the list of config files.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
diff --git a/cmake/app/boilerplate.cmake b/cmake/app/boilerplate.cmake
index b1907c6..ac628fa 100644
--- a/cmake/app/boilerplate.cmake
+++ b/cmake/app/boilerplate.cmake
@@ -405,7 +405,7 @@
     # Need path in order to check if it is absolute.
     get_filename_component(CONF_FILE_NAME ${CONF_FILE} NAME)
     get_filename_component(CONF_FILE_DIR ${CONF_FILE} DIRECTORY)
-    if(${CONF_FILE} MATCHES "prj_(.*).conf")
+    if(${CONF_FILE_NAME} MATCHES "prj_(.*).conf")
       if(NOT IS_ABSOLUTE ${CONF_FILE_DIR})
         set(CONF_FILE_DIR ${APPLICATION_SOURCE_DIR}/${CONF_FILE_DIR})
       endif()