Added library reference em_libref to EMD interface.

Occasionally dynamically drivers suffer from not being to refer to
content in the C library. Therefore I added a mechanism to allow
a C library for a certian target to define a symbol that will be
handed over to dynamic drivers for that target. Then the drivers
can use their refernce to that symbol to access content in the C
library.

libsrc/em/em-kernel.s

@@ -4,7 +4,7 @@
 ; Common functions of the extended memory API.
 ;
 
-        .import         return0
+        .import         return0, em_libref
         .importzp       ptr1
 
         .include        "em-kernel.inc"
@@ -55,6 +55,15 @@ _em_install:
         dey
         bpl     @L0
 
+; Set the library reference
+
+        ldy     #EMD_HDR::LIBREF
+        lda     #<em_libref
+        sta     (ptr1),y
+        iny
+        lda     #>em_libref
+        sta     (ptr1),y
+
 ; Copy the jump vectors
 
         ldy     #EMD_HDR::JUMPTAB