In gptboot, don't assume a partition number is a single digit, 1-9. GPT

In gptboot, don't assume a partition number is a single digit, 1-9.  GPT
partitions can have 128 partitions, so parse contiguous digits and then
validate that the number is between 1-128 inclusive.

I'm not sure 128 is a hard limit in the GPT standard, but it's the common
number in use, and it's a better upper limit than 9.

(cherry picked from commit 42e08952bb84aa86dfdcc7b80c95b58e515bdce7)