u-boot,at91bootstrap: fix incorrect HOSTCPPFLAGS variable

u-boot,at91bootstrap: fix incorrect HOSTCPPFLAGS variable

This would should up as `$$(HOSTCPPFLAGS)` in the host CFLAGS.
```
make --jobserver-fds=3,4 -j -C <openwrt>/build_dir/target-arm_cortex-a8+vfpv3_musl_eabi/u-boot-A10-OLinuXino-Lime/u-boot-2017.07 CROSS_COMPILE=arm-openwrt-linux-muslgnueabi- DTC="<openwrt>/build_dir/target-arm_cortex-a8+vfpv3_musl_eabi/linux-sunxi_cortexa8/linux-4.9.76/scripts/dtc/dtc" HOSTCC="gcc" HOSTCFLAGS='-O2 -I<openwrt>/staging_dir/host/include -I<openwrt>/staging_dir/host/usr/include -I<openwrt>/staging_dir/hostpkg/include -I<openwrt>/staging_dir/target-arm_cortex-a8+vfpv3_musl_eabi/host/include $$(HOSTCPPFLAGS)' HOSTLDFLAGS="" BL31=<openwrt>/staging_dir/target-arm_cortex-a8+vfpv3_musl_eabi/image/bl31.bin
```

And then it would complain with:
```
 /bin/sh: 1: HOSTCPPFLAGS: not found
```

Also, HOSTCPPFLAGS does not exist.
The correct var is HOST_CPPFLAGS.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>