This adds support for evaluating syscfg values as expressions. To make
it compatible with existing code, syscfg value is only evaluated as
expression if its type is explicitly set to "expr", i.e.:

syscfg.defs:
  FOO:
    description: ...
    type: expr
    value: 1 + 2 + 3

Following tokens are allowed in expressions:
- literals (integers and strings)
- identifiers (references to other syscfg values)
- parentheses
- binary operators (arthmetic, relational and boolean)
- unary operator (boolean negation)
- built-in function calls

Most of operators support only integer values. Strings are supported by
"==" and "!=" only.

Available built-in functions are:
- min(a,b) - returns lesser of "a" and "b"
- max(a,b) - returns greater of "a" and "b"
- in_range(v,a,b) - returns if "v" is inside [a,b] range
- clamp(v,a,b) - clamps "v" to be inside [a,b] range
- ite(v,a,b) - if-then-else, returns "a" if "v", otherwise returns "b"
- in_set(v,...) - returns if "v" is one of remaining arguments

Note: all arguments to built-in functions shall be integer only, except
for "a" and "b" in ite() and all arguments in in_set().

Except for defunc settings since those are not used anyway.

Some settings may reference values with MYNEWT_VAL_ prefix - this was
done to actually evaluate them as expression in preprocessor.
We can fix those and emit warning instead of failing.

If syscfg value is a choice, it's evaluated as an identifier so we need
to add special handling in such case - if identifier is evaluated
directly "from" entry, we should first check if value is a valid choice.

This adds function "raw" that expects a single string argument and will
return that argument contents as raw value, i.e. syscfg value will be
exactly that string without quotes.

This allows to differentiate empty strings (i.e. #define XXX "") and
empty values (i.e. #undef XXX).

this moves evaluator to separate package which interfaces with syscfg
via interface. this removes direct dependency on syscfg package.