The armv7m_load_kernel() function takes a mem_size argument which it
expects to be the size of the memory region at guest address 0.  (It
uses this argument only as a limit on how large a raw image file it
can load at address zero).

Instead of hardcoding this value, find the RAMInfo corresponding to
the 0 address and extract its size.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210215115138.20465-20-peter.maydell@linaro.org