tfa-layerscape: fix fiptool host build

tfa-layerscape: fix fiptool host build

fiptool is a host tool, used in a firmware generation pipeline, but it's
not treated as such, leading to the build breakage on the hosts which
don't have {Open,Libre}SSL dev package installed:

 In file included from fiptool.h:16:0,
                 from fiptool.c:19:
		 fiptool_platform.h:18:27: fatal error: openssl/sha.h:
		 No such file or directory
		  #  include <openssl/sha.h>

So this patch promotes fiptool into the host tool with proper host
include and library paths under STAGING_DIR.

Ref: https://github.com/openwrt/openwrt/pull/2267
Signed-off-by: Petr Štetiar <ynezz@true.cz>