Filesystem Size Used Available Use% Mounted on
devtmpfs 245.1M 0 245.1M 0% /dev
/dev/mmcblk0p3 511.7M 88.0K 511.6M 0% /config
rootfs on / type rootfs (rw,relatime)
proc on /proc type proc (rw,relatime)
devtmpfs on /dev type devtmpfs (rw,relatime,size=251008k,nr_inodes=62752,mode=755)
sysfs on /sys type sysfs (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
/dev/mmcblk0p3 on /config type vfat (rw,sync,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
devpts on /dev/pts type devpts (rw,relatime,gid=5,mode=620)
As it seems the only persistent storage is device /dev/mmcblk0p3 mounted on /config.
Therefore cgminer.conf and passwords remain on the system during upgrades&reboots.
One would have to modify the firmware file.