br-ext: Rename OP-TEE packages with _ext suffix

Rename OP-TEE packages in br-ext with a _ext suffix so that
they do not conflict with the OP-TEE packages from Buildroot.

This change essentially renames Buildroot configuration switches
BR2_OPTEE_* to BR2_OPTEE_*_EXT_* without changing the current build
process.

This change is needed for OP-TEE build environment to synchronize with
Buildroot GIT version [1] or later in the Buildroot integration tree.
[1] points where OP-TEE packages were introduced in Buildroot,
right after 2019.02 is released. This was tested with all Buildroot
release tags from 2018.11 to 2020.02, and recent 2020.05-rc1.

Link: [1] https://git.buildroot.net/buildroot/commit/?id=da49312af9e0c3e7f6df50e7d6f77e1fa2921b7a


Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>