Build stable releases 3.1.0 to 3.8.0

If you have a recent enough version of the Google repo tool (>= 2.0.0) and follow the normal build procedure at “Get and build the solution”, you will likely get an error during repo init with the following OP-TEE versions and platforms:

• 3.1.0 to 3.5.0: qemu_v8.xml, rpi3.xml

• 3.6.0 to 3.8.0: default.xml (QEMU), qemu_v8.xml, rpi3.xml

The typical error message is:

$ repo init -u https://github.com/OP-TEE/manifest.git -m qemu_v8.xml -b 3.3.0
[...]
fatal: manifest 'qemu_v8.xml' not available
fatal: <linkfile> invalid "src": ../toolchains/aarch64/bin/aarch64-linux-gnu-gdb: bad component: ..

The workaround is to checkout repo version 1.13.9 manually:

$ repo init -u https://github.com/OP-TEE/manifest.git -m qemu_v8.xml -b 3.3.0
# Above error occurs, ignore it
$ (cd .repo/repo; git checkout v1.13.9)
$ repo init -u https://github.com/OP-TEE/manifest.git -m qemu_v8.xml -b 3.3.0
# Should not error out. Then proceed with 'repo sync' and build.