[config] Enable PXE commands by default only on pcbios architecture

The commands bring in UNDI and thus real-mode code, so they cannot be
used on EFI.

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

src/config/defaults/pcbios.h

 #define IMAGE_BZIMAGE		/* Linux bzImage image support */
 #define IMAGE_COMBOOT		/* SYSLINUX COMBOOT image support */
 
+#define	PXE_CMD			/* PXE commands */
+
 #define	SANBOOT_PROTO_ISCSI	/* iSCSI protocol */
 #define	SANBOOT_PROTO_AOE	/* AoE protocol */
 

src/config/general.h

 #define LOGIN_CMD		/* Login command */
 #undef	TIME_CMD		/* Time commands */
 #undef	DIGEST_CMD		/* Image crypto digest commands */
-#define	PXE_CMD			/* PXE commands */
+//#undef	PXE_CMD			/* PXE commands */
 
 /*
  * Obscure configuration options