gptfdisk: use TARGET_CONFIGURE_OPTS properly, use TARGET_MAKE_ENV

$(TARGET_CONFIGURE_OPTS) should be passed as $(MAKE) argument, not in
the environment. We can then remove the redefined CC and LD variables
that were useless. We also take this opportunity to pass
$(TARGET_MAKE_ENV) in the environment of make.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>