cmake: Support UTF-8 characters in Kconfig values

We are using the CMake command 'file(STRINGS' (which defaults to only
decoding ASCII) to read Kconfig configurations. When UTF-8 characters
are detected they are treated as newlines.

This behaviour has caused issues when users have input UTF-8
characters into Kconfig values. E.g. USB device descriptor strings.

This commit adds the flag 'ENCODING "UTF-8"' to the 'file(STRINGS'
command so that UTF-8 characters are correctly passed through the
build system.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>