core/conversions: Added string to unsigned integer and long conversions

Added functions that convert a string to an unsigned integer or unsigned long.
A couple of unit test were also created to test the routines. The reasons for
adding these conversion utilities (and hopefully eventually more) are as
follows:

  * Conversion routines are functionally contained with consistent and
    better error checking
  * The function names offer a better description of what is happening
  * It encourages code reuse for easier bug fixing at a single source
  * It's simpler to use
  * It's unit testable

For instance, currently in a lot of places when converting to an integer or
similar the "sscanf" function is used. When using "sscanf" it may not be
immediately clear what's happening as it lacks semantic naming. Limited error
checking is usually done as well. For example, most of the time a check is done
to make sure the value converted, but does not check for overflows or negative
valued conversions when converting unsigned numbers.

Why use/wrap "strtoul" and not "sscanf" then? Primarily, it lacks some of the
built in error handling that "strtoul" has. For instance "strtoul" contains
overflow checks. Less so, but can still factor as reasons, "sscanf" is slightly
more complex in its use. And maybe a bit controversial, but it may be ("big if")
potentially slower than "strtoul" in some cases.

Change-Id: If7eaca4a48f8c7b89cc8b5a1f4bed2852fca82bb

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