binary_helpers: Rename bv_get_value to reflect it's length checking

binary_helpers: Rename bv_get_value to reflect it's length checking

Rename the bv_get_value() routine to bv_get_value_len() to better reflect
that this implementation checks the input data image's length. A version
which doesn't check length will get introduced soon.

[ gsi: use a shorter name than in the initial submission ]