tf-a/qemu v8: Change LOG_LEVEL to 40 when DEBUG=1

With LOG_LEVEL=50 and DEBUG=1, BL31 becomes too big, with LOG_LEVEL=40
we will miss a few debug prints, but it will at produce a valid binary
and more importantly, you'll get an elf-file with all debug symbols,
i.e., it's GDB makes up for the missing prints.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>