libavb: Avoid conflict with system-provided crc32 symbol.

Some environments define crc32() themselves which leads to a conflict
in avb_crc32.c where we define the crc32() function. Fix this by
prefixing with iavb_.

Also spotted that avb_rsa.c had two internal functions that weren't
marked as static functions. Fix this and future problems like this by
using -Wmissing-prototypes (except don't use it together with gtest
because it doesn't work there). Additionally prefix the identifiers
used in avb_rsa.c since they are very common (Key struct for example).

We could change all internal identifiers (function names, struct
names, macros) to be prefixed (with iavb_, IAvb, IAVB_ respectively)
but this CL does not do that.

Test: New unit tests and all unit tests pass.
Test: Manually tested on UEFI-based boot loader.
Bug: 63072777
Change-Id: Ie894679a64cd9c305b32d610dae2aae1d5fd486e