Accton powerpc platforms: Sync u-boot sys_eeprom to latest code

The platforms are including:
- AS4600_54T
- AS5600_52X
- AS5610_52X
- AS5710_54X
- AS6700_32X
- AS6701_32X
- AS6710_32X

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
u-boot sys_eeprom: Revise 0x29 TLV name to 'ONIE Version'

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
busybox sys_eeprom: Revise 0x29 TLV name to 'ONIE Version'

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Accton AS7700_32X: Add to support ONIE

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Accton AS7702_32X: Add to support ONIE

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Add support for upstream syslinux

To insulate ONIE build from syslinux version changes in different Linux
distro's, we're pulling in a specific `syslinux` version from upstream.

The version selected is the latest from the 4.x series which is what
ONIE has been using till now. The exact version is - `4.07`.

The upstream package already comes bundled with pre-built binaries which
we are using directly (hence no syslinux source build needed).

The `debian-prepare-build-host` target has the `syslinux-common` package
removed as it's no longer needed.

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