include/rootfs.mk: remove boot directory

Currently every file in boot directory is copied over target /boot on
root file system and is usually inaccessible because appropriate boot
file system is mounted on top of it. Therefore remove /boot, which in
result will also save space on target root file system.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>
(backported from 31075313bf)

include/rootfs.mk

@@ -90,6 +90,7 @@ define prepare_rootfs
 	rm -f $(1)/usr/lib/opkg/lists/*
 	rm -f $(1)/usr/lib/opkg/info/*.postinst*
 	rm -f $(1)/var/lock/*.lock
+	rm -rf $(1)/boot
 	$(call clean_ipkg,$(1))
 	$(call mklibs,$(1))
 endef