build system: do not rebuild ash and hush on any change to any .c file

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush.c b/shell/hush.c
index e64c923..29ff3c4 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -128,9 +128,7 @@
 # define USE_FOR_MMU(...)
 #endif
 
-#define SKIP_definitions 1
-#include "applet_tables.h"
-#undef SKIP_definitions
+#include "NUM_APPLETS.h"
 #if NUM_APPLETS == 1
 /* STANDALONE does not make sense, and won't compile */
 # undef CONFIG_FEATURE_SH_STANDALONE