pxeboot: Allow a different default loader for pxeboot

Sometimes you need / want a different boot loader than loader_lua for
pkeldr. Provide an option to get either the 4th one or the simp one.

MFC After:		3 days
Sponsored by:		Netflix
Reviewed by:		markj
Differential Revision:	https://reviews.freebsd.org/D46213

stand/i386/Makefile

@@ -20,6 +20,11 @@ SUBDIR.${MK_LOADER_PXEBOOT}+=	pxeldr
 
 SUBDIR.${MK_LOADER_ZFS}+=	zfsboot gptzfsboot
 
-SUBDIR_DEPEND_pxeldr+=	loader_${LOADER_DEFAULT_INTERP}
+.if defined(PXEBOOT_DEFAULT_INTERP)
+L=${PXEBOOT_DEFAULT_INTERP}
+.else
+L=${LOADER_DEFAULT_INTERP}
+.endif
+SUBDIR_DEPEND_pxeldr+=	loader_${L}
 
 .include <bsd.subdir.mk>

stand/i386/pxeldr/Makefile

@@ -22,7 +22,11 @@ CFLAGS+=-DALWAYS_SERIAL
 
 CFLAGS+=-I${BOOTSRC}/i386/common
 
+.if defined(PXEBOOT_DEFAULT_INTERP)
+L=${PXEBOOT_DEFAULT_INTERP}
+.else
 L=${LOADER_DEFAULT_INTERP}
+.endif
 LOADERBIN= ${BOOTOBJ}/i386/loader_${L}/loader_${L}.bin
 
 CLEANFILES+= ${BOOT}.tmp