package/libnss: use correct CFLAGS and LDFLAGS

package/libnss: use correct CFLAGS and LDFLAGS

Currently libnss is configured with BUILD_OPT=1 and due to that that
it doesn't take into account Buildroot TARGET_CFLAGS nor
TARGET_LDFLAGS resulting in pre-chosen optimizations and debugging
symbols being used.

We can't pass TARGET_CFLAGS through CFLAGS or similar otherwise other
internal libnss one will be overwritten (i.e. -fPIC), so we prefer to
append TARGET_CFLAGS at the end of Linux.mk as well as TARGET_LDFLAGS
according to internal libnss Makefile system's names. And obviously
remove BUILD_OPT=1 from BUILD_VARS.

This reveals hidden bugs when building with -Os due to bogus
uninitialized warnings from gcc turned into errors because of -Werror,
so we explicitly pass NSS_ENABLE_WERROR=0 to avoid the use of -Werror.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[Thomas: drop the libnss patches, and pass NSS_ENABLE_WERROR instead]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>