Blackfin: disable NetBSD bootm support by default

There is no Blackfin/NetBSD port, so enabling support for it by default
doesn't make any sense.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/include/asm-blackfin/config-pre.h b/include/asm-blackfin/config-pre.h
index 1170a2a..4531519 100644
--- a/include/asm-blackfin/config-pre.h
+++ b/include/asm-blackfin/config-pre.h
@@ -71,6 +71,9 @@
 # define BFIN_BOOT_SPI_SSEL 1
 #endif
 
+/* There is no Blackfin/NetBSD port */
+#undef CONFIG_BOOTM_NETBSD
+
 /* We rarely use interrupts, so favor throughput over latency */
 #define CONFIG_BFIN_INS_LOWOVERHEAD