Merge TARGET_CONFIGURE_ENV into TARGET_CONFIGURE_OPTS

TARGET_CONFIGURE_ENV defines CFLAGS, LDFLAGS, CXXFLAGS and FCFLAGS,
separatly from all other variables that are part of
TARGET_CONFIGURE_OPTS. This is useless and not consistent with the
HOST_CONFIGURE_ variables, therefore we merge TARGET_CONFIGURE_ENV
into TARGET_CONFIGURE_OPTS and fix the few users of
TARGET_CONFIGURE_ENV.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>