Copy actual partition size instead of hardcoded 16MB

Created by: davidschrammel

Changed the size of the copied partition as discussed in #169 (closed). I have tested this on my setup and it seems to be working as expected.

Should I also include the updated bootrom.h and bootrom.sv files in this PR or do you want to build them yourself? Since these a binary blobs, it makes sense that an official maintainer creates these.

At some point we might also want to change the name of gpt_find_boot_partition, since it does not just find but also copy the boot partition. Also, the size argument is now unused: https://github.com/openhwgroup/cva6/blob/7760227f887c59a2ae1b89e70fb20582a2d5c5b3/fpga/src/bootrom/src/gpt.c#L7 https://github.com/openhwgroup/cva6/blob/0bbc3cb5cf43cd3bf77d3872e46dd0d6ededf28f/openpiton/bootrom/linux/src/main.c#L14 https://github.com/openhwgroup/cva6/blob/0bbc3cb5cf43cd3bf77d3872e46dd0d6ededf28f/fpga/src/bootrom/src/main.c#L11