elftosb: use HOST_CONFIGURE_OPTS instead of HOST_MAKE_ENV

HOST_CONFIGURE_OPTS allows to pass the correct CC, CXX, CFLAGS,
CXXFLAGS, etc. For example, it allows the elftosb build process to
properly use ccache when available.

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