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

shell/ash.c

@@ -6471,7 +6471,7 @@ tryexec(char *cmd, char **argv, char **envp)
 				run_current_applet_and_exit(argv);
 			}
 			/* re-exec ourselves with the new arguments */
-			execve(CONFIG_BUSYBOX_EXEC_PATH, argv, envp);
+			execve(bb_busybox_exec_path, argv, envp);
 			/* If they called chroot or otherwise made the binary no longer
 			 * executable, fall through */
 		}

shell/hush.c

@@ -1392,7 +1392,7 @@ static void pseudo_exec_argv(char **argv)
 			}
 			/* re-exec ourselves with the new arguments */
 			debug_printf_exec("re-execing applet '%s'\n", argv[0]);
-			execvp(CONFIG_BUSYBOX_EXEC_PATH, argv);
+			execvp(bb_busybox_exec_path, argv);
 			/* If they called chroot or otherwise made the binary no longer
 			 * executable, fall through */
 		}

shell/msh.c

@@ -3062,7 +3062,7 @@ static const char *rexecve(char *c, char **v, char **envp)
 			/* We have to exec here since we vforked.  Running
 			 * run_applet_and_exit() won't work and bad things
 			 * will happen. */
-			execve(CONFIG_BUSYBOX_EXEC_PATH, v, envp);
+			execve(bb_busybox_exec_path, v, envp);
 		}
 	}