stm32f205: Create armv7m object without using armv7m_init()

Switch the stm32f205 SoC to create the armv7m object directly
rather than via the armv7m_init() wrapper. This fits better
with the SoC model's very QOMified design.

In particular this means we can push loading the guest image
out to the top level board code where it belongs, rather
than the SoC object having a QOM property for the filename
to load.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-11-git-send-email-peter.maydell@linaro.org