Add support for including files in linkerscript

Files can now be included with the following syntax:

    INCLUDE "path.link"

The maximum include depth is 5.

Fixed linkerscript parser and lexer error messages so that they are more
informative (show file and line of the error).

Man page updated.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/link/assign.c

@@ -445,6 +445,7 @@ AssignSections(void)
 	 */
 
 	if (tzLinkerscriptName) {
+		script_InitSections();
 		script_Parse(tzLinkerscriptName);
 	}