bootstat: alias underline to space in bit error rate handling

When we are matching existing known boot reasons, we should try with
compliant underlines first, then again with underlines replaced with
spaces.  Replace references to Ber with BitError for maintenance
clarity.  Replace helper functions with C++11 lambdas.

Test: boot_reason_test.sh
Bug: 63736262
Change-Id: I91b865013513526a55a85523080c7127f198968c