[pxe] Remove startpxe and stoppxe commands from default builds

These commands exist primarily for debugging and are not generally
useful, so save 137 bytes by removing them by default.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/defaults/pcbios.h

 
 #define PXE_STACK		/* PXE stack in iPXE - required for PXELINUX */
 #define PXE_MENU		/* PXE menu booting */
-#define	PXE_CMD			/* PXE commands */
 
 #define	SANBOOT_PROTO_ISCSI	/* iSCSI protocol */
 #define	SANBOOT_PROTO_AOE	/* AoE protocol */

src/config/general.h

 #undef	DIGEST_CMD		/* Image crypto digest commands */
 #undef	LOTEST_CMD		/* Loopback testing commands */
 #undef	VLAN_CMD		/* VLAN commands */
-//#undef	PXE_CMD			/* PXE commands */
+#undef	PXE_CMD			/* PXE commands */
 
 /*
  * Error message tables to include