uboot-fritz4040: Fix build with HOSTCFLAGS

uboot-fritz4040: Fix build with HOSTCFLAGS

When we provide the HOSTCFLAGS to the U-Boot build it will fail because
it can not find the u-boot provided header files any more.
Just overwrite and not append the package specific configuration on top
of the configuration provided by u-boot.mk.
uboot-fritz4040 is based on U-Boot 2012.07 and this problem is probably
similar to the problem seen with the lantiq and ar71xx u-boot build.

Fixes: df9781a4205 ("u-boot,at91bootstrap: fix incorrect HOSTCPPFLAGS variable")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>