Build bootloader kernel seperate from skiboot

We want the kernel to be placed in it's own BOOTKERNEL partition on the
flash. In that case we don't want it linked into the skiboot image,
instead it should be incorporated independently in to the PNOR image.

This is made the default for Palmetto, Habanero and Firestone configs.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/configs/firestone_defconfig b/openpower/configs/firestone_defconfig
index c9de842..570d9df 100644
--- a/openpower/configs/firestone_defconfig
+++ b/openpower/configs/firestone_defconfig
@@ -23,7 +23,7 @@
 
 # skiboot requirements
 BR2_TARGET_SKIBOOT=y
-BR2_TARGET_SKIBOOT_EMBED_PAYLOAD=y
+BR2_TARGET_SKIBOOT_EMBED_PAYLOAD=n
 
 # petitboot requirements
 BR2_ENABLE_LOCALE_PURGE=y