make busybox --install work even if /proc/self/exe doesn't exist

# size busybox_old busybox_unstripped
   text    data     bss     dec     hex filename
 680095    2704   15648  698447   aa84f busybox_old
 680099    2704   15648  698451   aa853 busybox_unstripped

libbb/execable.c

@@ -65,7 +65,7 @@ int exists_execable(const char *filename)
  */
 int bb_execvp(const char *file, char *const argv[])
 {
-	return execvp(find_applet_by_name(file) ? CONFIG_BUSYBOX_EXEC_PATH : file,
+	return execvp(find_applet_by_name(file) ? bb_busybox_exec_path : file,
 					argv);
 }
 #endif