qemu: Simplify the image size calculation

qemu: Simplify the image size calculation

Patch introduce the macro NS_IMAGE_MAX_SIZE to simplify the image size
calculation. Use of additional parenthesis removes the possibility of
improper calculations due nested macro expansion for subtraction.
In case of platforms with DRAM window over 32bits, patch also removes
potential problems with type casting, as meminfo.image_size is uint32_t
but macro calculations were done in 64bit space.

Signed-off-by: Radoslaw Biernacki <radoslaw.biernacki@linaro.org>
Change-Id: I2d05a2d9dd6000dba6114df53262995cf85af018