sandbox: Correct compiler warnings in cmd_bootm/cmd_ximg

sandbox: Correct compiler warnings in cmd_bootm/cmd_ximg

Correct the following warnings found with sandbox when compression
is enabled.

cmd_bootm.c: In function 'bootm_load_os':
cmd_bootm.c:443:11: warning: passing argument 4 of 'lzop_decompress' from incompatible pointer type [enabled by default]
/usr/local/google/c/cosarm/src/third_party/u-boot/files/include/linux/lzo.h:31:5: note: expected 'size_t *' but argument is of type 'uint *'
cmd_ximg.c: In function 'do_imgextract':
cmd_ximg.c:225:6: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
cmd_ximg.c:225:14: warning: 'hdr' may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>