tools/mkimport: Add system map to mkimport script.

The `make export` target from the Nuttx repository correctly packs the
System.map file. This change ensure that ./tools/mkimport.sh extracts
this file correctly into the `import`correctly.

tools/mkimport.sh

diff --git a/tools/mkimport.sh b/tools/mkimport.sh
index 2173f9b..6af33b0 100755
--- a/tools/mkimport.sh
+++ b/tools/mkimport.sh
@@ -162,7 +162,7 @@
 
 # Move the .config file in place in the import directory
 
-SFILES=".config"
+SFILES=".config System.map"
 for file in ${SFILES}; do
 	if [ -f "${EXPORTDIR}/${file}" ]; then
 		cp -a ${EXPORTDIR}/${file} ${IMPORTDIR}/${file} || \