Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Similar to C counterparts, use OPTEE_CLIENT_EXPORT as a standard path to
export pre-built optee-client libraries and headers.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Similar to C counterparts, use TA_DEV_KIT_DIR as a standard path to
export pre-built TA libraries and headers.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Refactor OP-TEE rust examples build infrastructure to rather use
standard environment variables like:
- CROSS_COMPILE
- TA_DEV_KIT_DIR

With that remove any custom OPTEE_* environment variables.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Rather than automatically building OP-TEE OS and OP-TEE Client, we
should ask users to build them for their particular platforms using
whichever OP-TEE release they are targeting.

Users just have to export paths for OP-TEE OS build artifacts via
TA_DEV_KIT_DIR and for OP-TEE Client via OPTEE_CLIENT_EXPORT. This is
the common practice followed by C counterparts like optee_test and
optee_examples. Along with that it make OP-TEE rust examples compatible
with build systems like Yocto etc.

Regarding toolchains, we don't need to fetch them explicitly but rather
users can use the distro cross compiler or the one they have installed
on a particular location via CROSS_COMPILE environment variable.

With that build command simplifies to just following:

$ make CROSS_COMPILE=aarch64-linux-gnu- \
       TA_DEV_KIT_DIR=<optee_os>/out/arm-plat-vexpress/export-ta_arm64/ \
       OPTEE_CLIENT_EXPORT=<optee_client>/out/export/

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Since we don't have a similar linker script pre-processing capability in
rust build script as we have with C Makefile. So manually fix following
build error seen with older GCC linker while parsing linker scripts:

aarch64-linux-gnu-ld.bfd:ta.lds:3: ignoring invalid character `#' in expression

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

arm-none-linux-gnueabihf-ld.bfd reports EABI version mismatch issue,
drop that error via adding "--no-warn-mismatch" linker flag.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Extend CI to build OP-TEE rust examples for both Arm 64-bit and Arm
32-bit mode.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>