boot/uboot: fix build of old U-Boot versions

boot/uboot: fix build of old U-Boot versions

Since commit 0dca644e443c8f28bfe5aa1c98201dc3b49e1165 ("boot/uboot: fix
missing host-openssl for i.MX28 target"), we pass HOSTCFLAGS/HOSTLDFLAGS
in the environment of U-Boot when building. This is needed to allow
U-Boot to find the OpenSSL headers/libraries when they are
needed. Unfortunately, this breaks the build with old U-Boot versions as
the U-Boot Makefiles were not designed to have HOSTCFLAGS passed in,
which causes some important CFLAGS from the U-Boot build system to be
ignored.

As suggested by Arnout, we pass the HOST_CFLAGS inside HOSTCC directly,
which allows to pass the CFLAGS without overriding the internal U-Boot
CFLAGS.

Tested with an old U-Boot (2012.10), and a modern U-Boot in a
configuration that needs OpenSSL.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>