example-config: update OPENXT_GIT_MIRROR syntax and add OPENXT_GIT_PROTOCOL

Remove leading git:// from OPENXT_GIT_MIRROR and create new variable
OPENXT_GIT_PROTOCOL to populate the protocol= field used by the bitbake
fetcher.

OXT-10
do_build.sh: set OPENXT_GIT_PROTOCOL and OPENXT_BRANCH in local.conf

OPENXT_BRANCH is used for branch= flag for bitbake's git fetcher.
This was renamed from previously unused XENCLIENT_BRANCH.

OPENXT_GIT_PROTOCOL is used for protocol= flag.

OXT-10
do_build.sh: incorporate OPENXT_GIT_PROTOCOL where necessary for git URLs.

OXT-10
do_build.sh: fixup do_oe_syncui_copy() to handle OE version changes.

Reported-by: Brendan Kerrigan <kerriganb@ainfosec.com>

OXT-10
Merge pull request #36 from cjp256/oxt-10-v2

[OXT-10] v2