package infra: add -static to C/CXXFLAGS

Generic infra packages might not use LDFLAGS at all so add -static for
static builds to CFLAGS and CXXFLAGS too.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>