icu: add an option to add a custom data library file

[Thomas:
 - use one single BR2_PACKAGE_ICU_CUSTOM_DATA_PATH string option
   instead of a boolean option + a string option, which always causes
   a lot of issues with random configurations that enable the boolean
   option, but do not provide a valid value for the string option.
 - enclose the definition of ICU_COPY_CUSTOM_DATA into the condition.]

Signed-off-by: Johan Derycke <johan.derycke@barco.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>