Add Accton AS4600_54T to support ONIE

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Add PLATFORM_STRING definition for u-boot to check platform name

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Allow to customize u-boot identity string for machine vendor

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>