hw/arm/boot: Factor out "direct kernel boot" code into its own function

hw/arm/boot: Factor out "direct kernel boot" code into its own function

Factor out the "direct kernel boot" code path from arm_load_kernel()
into its own function; this function is getting long enough that
the code flow is a bit confusing.

This commit only moves code around; no semantic changes.

We leave the "load the dtb" code in arm_load_kernel() -- this
is currently only used by the "direct kernel boot" path, but
this is a bug which we will fix shortly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20190131112240.8395-3-peter.maydell@linaro.org