Use CFLAGS_DEBUG which is already used for TARGET_CFLAGS to also pass
TARGET_CPPFLAGS.
This fixes build issues with toolchains that don't automatically search
for header files (e.g: sys/capability.h), such as stbgcc-4.8-1.5
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>