image: android: Fix possible build errors

As android_image.h uses types like u32, we need to include corresponding
headers in place. Otherwise the user will be forced to include those in
C file, or next build error can occur:

    include/android_image.h:32:5: error: unknown type name 'u32'
         u32 kernel_size; /* size in bytes */

Include required headers for data types used. While at it, remove
typedef struct, which is prohibited by kernel coding style, and fix the
comment.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>