cmake: fix improper use of separate_arguments

The use of `separate_arguments()` in extra_flags.cmake and
unittest.cmake prevents the use of proper CMake lists.

A CMake list can be specified on command line as:
`-DMY_LIST="itemA;itemB;itemC"`

Users are expected to be able to provide extra compile flags using CMake
lists, like `-DEXTRA_CFLAGS="--flag1;--flag2"` but also using space
separated lists, like: `-DEXTRA_CFLAGS="--flag1 --flag2"`.

However, the way FLAGS was passed to separate_arguments() would result
in a list being treated as extra arguments to the function and thus
result in a CMake error.

To allow the use of CMake lists, the length of the argument as a CMake
list is tested. This ensures that when the argument contains a
single `;` then the length is >1 and no conversion is needed.

This allows a user to freely choose between the two forms:
- `-DFLAGS="--flag1 --flag2"`
- `-DFLAGS="--flag1;--flag2"`

or when passing flags which themselves contains spaces, like:
- `-DFLAGS="--flag1 \"val1 val2\""`
- `-DFLAGS="--flag1;val1 val2"`

Note that the extra escaped qouting is no longer needed for proper CMake
lists as the `;` itself informs the system that spaces are to be kept as
spaces.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>