optee_client

Android use by default Soong Build System, *.mk files are deprecated
in favor of Android.bp [1].

All the logics present in android mk files have been ported to a
single Android.bp

optee_client.device.mk set the same default flags as we did in the
old Android *.mk files.

Example of configs in device/VENDOR/BOARD/device.mk:
```
include $(PATH_OPTEE_CLIENT)/optee_client.device.mk
PRODUCT_PACKAGES += \
    libteec \
    tee-supplicant
```

[1] https://source.android.com/docs/setup/build



Signed-off-by: Julien Masson <jmasson@baylibre.com>