imgmgr: Prevent `maybe-uninitialized` warning.
imgmgr: Prevent `maybe-uninitialized` warning.

gcc 5.4.1 reports the following warning when `BOOTUTIL_IMAGE_FORMAT_V2`
is enabled:

    Error: repos/apache-mynewt-core/mgmt/imgmgr/src/imgmgr.c: In function 'imgr_read_info':
    repos/apache-mynewt-core/mgmt/imgmgr/src/imgmgr.c:202:16: error: 'data_end' may be used uninitialized in this function [-Werror=maybe-uninitialized]
                 if (data_off + IMGMGR_HASH_LEN > data_end) {
                    ^

This is a spurious warning; `data_end` is always initialized at this
point.  The fix is to initialize `data_end` to 0 at the start of the
function.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core