ar71xx: Speed up caldata/eeprom handling

ar71xx: Speed up caldata/eeprom handling

Reading and writing to and from flash storage is slowed down
enormously by some functions which use a block size of 1.

This patch reworks the extraction scripts to be much faster and
efficient by reading and writing in possibly one big block.

This is based on the initial commit a69e101 for ipq40xx by
Christian Lamparter <chunkeey@gmail.com>.

Speed comparison @ TP-Link TL-WDR4300 (just manually) results
in a time reduction by three orders of magnitude (99.9 %).

> time dd if=/dev/mtd3 of=/lib/firmware/test-slow bs=1 count=4096 skip=4096
4096+0 records in
4096+0 records out
real    0m 15.85s
user    0m 0.06s
sys     0m 13.28s

> time dd if=/dev/mtd3 of=/lib/firmware/test-fast bs=4096 count=1 skip=4096 iflag=skip_bytes
1+0 records in
1+0 records out
real    0m 0.02s
user    0m 0.00s
sys     0m 0.02s

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>