flashmap: Add support for linker script generation

Flash map form bsp.yml could easily be inconsistent with linker scrip memory regions.

This change aims to allow usage of sysflash.h header generate by newt tool to be used by c preprocessor to automatically generate linker scrips that are consistent with flash description from bsp.yml.
This simply excludes part of the header so only constants are exported.

newt/flashmap/flashmap.go

@@ -245,12 +245,15 @@ func writeFlashMapHeader(w io.Writer, fm FlashMap) {
 	fmt.Fprintf(w, "#ifndef H_MYNEWT_SYSFLASH_\n")
 	fmt.Fprintf(w, "#define H_MYNEWT_SYSFLASH_\n")
 	fmt.Fprintf(w, "\n")
-	fmt.Fprintf(w, "#include \"flash_map/flash_map.h\"\n")
-	fmt.Fprintf(w, "\n")
 	fmt.Fprintf(w, "#define FLASH_AREA_COUNT %d\n", len(fm.Areas))
 	fmt.Fprintf(w, "\n")
+	fmt.Fprintf(w, "/* MYNEWT_SYSFLASH_ONLY_CONST is defined when this header is used for linker script generation */\n")
+	fmt.Fprintf(w, "#ifndef MYNEWT_SYSFLASH_ONLY_CONST\n")
+	fmt.Fprintf(w, "#include \"flash_map/flash_map.h\"\n")
+	fmt.Fprintf(w, "\n")
 	fmt.Fprintf(w, "%s", C_VAR_COMMENT)
 	fmt.Fprintf(w, "extern %s;\n", flashMapVarDecl(fm))
+	fmt.Fprintf(w, "#endif\n")
 	fmt.Fprintf(w, "\n")
 	fmt.Fprintf(w, "/* Flash map was defined in %s */\n", fm.PkgName)
 	fmt.Fprintf(w, "\n")