image loading: fix problem with offsets > 0x80000000

image loading: fix problem with offsets > 0x80000000

Fixes bug that prevented users from specifying a base address of
0x80000000 or higher in image commands (flash write_image, etm image,
xscale trace_image).

image.base_address is an offset from the start address contained in
the image file (if there is one), or from 0 (for binary files). As a
signed 32-bit int, it couldn't be greater than 0x7fffffff, which is a
problem when trying to write a binary file to flash above that
address. Changing it to a 64-bit long long keeps it as a signed
offset, but allows it to cover the entire 32-bit address space.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>