[shell] Enable "shell" command even when BANNER_TIMEOUT is zero

Setting BANNER_TIMEOUT to zero removes the only symbol reference to
shell.o, causing the "shell" command to become unavailable.

Add SHELL_CMD in config/general.h (enabled by default) which will
explicitly drag in shell.o regardless of the value of BANNER_TIMEOUT.

Reported-by: Julian Brost <julian@0x4a42.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/config.c

 #ifdef SYNC_CMD
 REQUIRE_OBJECT ( sync_cmd );
 #endif
+#ifdef SHELL_CMD
+REQUIRE_OBJECT ( shell );
+#endif
 #ifdef NSLOOKUP_CMD
 REQUIRE_OBJECT ( nslookup_cmd );
 #endif

src/config/general.h

 #define MENU_CMD		/* Menu commands */
 #define LOGIN_CMD		/* Login command */
 #define SYNC_CMD		/* Sync command */
+#define SHELL_CMD		/* Shell command */
 //#define NSLOOKUP_CMD		/* DNS resolving command */
 //#define TIME_CMD		/* Time commands */
 //#define DIGEST_CMD		/* Image crypto digest commands */