open-plc-utils: fix build with static musl

open-plc-utils: fix build with static musl

Pass TARGET_LDFLAGS to EXTRA_LDFLAGS to fix following issue:
/home/buildroot/autobuild/run/instance-3/output/build/host-gcc-final-7.3.0/build/arm-buildroot-linux-musleabihf/libgcc/../../../libgcc/config/arm/lib1funcs.S:1545: undefined reference to `raise'

Also pass TARGET_CFLAGS to EXTRA_CFLAGS and TARGET_CXXFLAGS to
EXTRA_CXXFLAGS and move all these variables to
OPEN_PLC_UTILS_MAKE_OPTS for readability

Fixes:
 - http://autobuild.buildroot.org/results/67bc5e7ac8ae1c49c035b022a394d2f746705cf2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>