conf: phyrk3288.inc: Rework for u-boot usage
conf: phyrk3288.inc: Rework for u-boot usage

- Remove barebox as 2nd stage bootloader
- Provide u-boot SPL binary
- Use u-boot as 1st & 2nd stage bootloader

Signed-off-by: Wadim Egorov <f42ccee66aacafbe10be12a603629f53944208c9@phytec.de>
Signed-off-by: Stefan Müller-Klieser <a3e08e35e4a42be768e7d0e76ae1ba321c2a66dd@phytec.de>

PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec