tools: spiffsgen.py: avoid reallocating byte array for each new block
tools: spiffsgen.py: avoid reallocating byte array for each new block

On large filesystems (~15 MB), this reduces execution time from
11s to 0.3s.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf