libteec/tee-supplicant: respect LDFLAGS set from distribution toolchain

Unify LFLAGS/LDFLAGS variable.

For example, on OpenEmbedded, it fixes the following QA error:
QA Issue:
No GNU_HASH in the elf binary: optee-client/usr/bin/tee-supplicant'
No GNU_HASH in the elf binary: optee-client/usr/lib/libteec.so.1.0' [ldflags]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>