u-boot: Make u-boot-aspeed-sdk the default

This makes the v2019.04-aspeed-openbmc branch the default for all aspeed
machines.

Existing machines have all modified their u-boot package to use the
older branch, so this patch should result in no change.

Change-Id: Ic4dd0bb973a16d3d658c340e7caf6a05128327bc
Signed-off-by: Joel Stanley <joel@jms.id.au>