Replace fixed values in CBMC Makefiles by definable values

Some unwinding values appearing in CBMC Makefiles are fixed values. For example, the following one in proof `aws_cryptosdk_enc_ctx_size`

```
UNWINDSET += aws_add_size_checked_varargs.0:5
```
should become a value defined somewhere as in
```
UNWINDSET += aws_add_size_checked_varargs.0:$(MAX_VARS_LEN)
```

Search the CBMC Makefiles for fixed unwinding values, starting with proof aws_cryptosdk_enc_ctx_size and the aws_add_size_checked_varargs.0:5 entry. Identify where configurable values are defined; done means the fixed unwinding values are replaced with definable values without changing the proof targets.

Written by the indexing model from the issue text.