iopsys-econet: Remove global UNIFIED_PLATFORM flag

TCSUPPORT_FH_UNIFIED_PLATFORM flag was used as temporar fix for flash
mounting. As suggested by Eocnet this flag should not be set global.
Moved to kernel mtd driver instead.

Signed-off-by: Bartlomiej Grzeskowiak <bartlomiej.grzeskowiak@iopsys.eu>