utils: tip_check_csum() uses local checksum file

.full.sum file in the local cache contains the local checksum for this file,
this checksum is calculated by tip_write_csum(), therefore, there is no need to
calculate this checksum again from the image file in the cache, use this
checksum content instead.

if .full.sum is not available, then cache is inconsistent, tip_check_csum()
is called after checking if image file exists, raise an exception.

checksum could mismatch in two situations:

a) new image version (checksum is different)
b) image is corrupted

in both cases, a new fresh image needs to be retrieved.

this speeds up checksum validation.

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