tools/mkimport.sh: enrich help message

This enrichs the help message for `tools/mkimport.sh`

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

tools/mkimport.sh

@@ -19,7 +19,23 @@
 
 # Get the input parameter list
 
-USAGE="USAGE: $0 [-d] [-z] [-l <ext>] -x <export-path>"
+USAGE="
+
+DESCRIPTION:
+
+  Preparing folder 'import/' using the export package from NuttX kernel
+  build.
+
+USAGE: $0 [-d] [-z] [-l <ext>] -x <export-package>
+
+Where:
+
+  -d enable debugging output
+  -h show this help message
+  -l <ext> extension of library file (default is .a)
+  -x <export-package> export package file name
+  -z expect gzip compressed tar ball
+"
 unset EXPORT
 unset TGZ
 LIBEXT=.a
@@ -41,7 +57,7 @@ while [ ! -z "$1" ]; do
 			TGZ=y
 			;;
 		-h )
-			echo $USAGE
+			echo "$USAGE"
 			exit 0
 			;;
 		* )