[RX51] Export u-boot kernel image with flasher
This properly exports the uImage kernel, which is used by u-boot, when running 'flasher export'. Note the change to the uImage name to follow the pattern "uImage-$flavor", which in the case of the RX 51 is "uImage-nokia-rx51". This pattern is used by the 'export' function in 'flasher' to match files for exporting.
@MartijnBraam FYI