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/ash.c b/shell/ash.c
index 1f8f90a..cfd8154 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -50,9 +50,7 @@
 # define CLEAR_RANDOM_T(rnd) ((void)0)
 #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