Add avb_div_by_10() sysdep operation.

On some low-level 32-bit embedded systems, performing uint64_t division
(which calls a libc function) might not be supported by the CPU, and
creates linking issues when using gcc. Instead, bootloaders like uboot
provides extra functions (do_div()) as software solutions to perform the
divisions. This commit provides a way to call that do_div().

TEST=Unit tests.

Change-Id: Ia00c4998985a04fba6b85495d993dd86ed32801a