crypto: vmx - Avoid weird build failures

In the vmx crypto Makefile we assign to a variable called TARGET and
pass that to the aesp8-ppc.pl and ghashp8-ppc.pl scripts.

The variable is meant to describe what flavour of powerpc we're
building for, eg. either 32 or 64-bit, and big or little endian.

Unfortunately TARGET is a fairly common name for a make variable, and
if it happens that TARGET is specified as a command line parameter to
make, the value specified on the command line will override our value.

In particular this can happen if the kernel Makefile is driven by an
external Makefile that uses TARGET for something.

This leads to weird build failures, eg:
  nonsense  at /build/linux/drivers/crypto/vmx/ghashp8-ppc.pl line 45.
  /linux/drivers/crypto/vmx/Makefile:20: recipe for target 'drivers/crypto/vmx/ghashp8-ppc.S' failed

Which shows that we passed an empty value for $(TARGET) to the perl
script, confirmed with make V=1:

  perl /linux/drivers/crypto/vmx/ghashp8-ppc.pl  > drivers/crypto/vmx/ghashp8-ppc.S

We can avoid this confusion by using override, to tell make that we
don't want anything to override our variable, even a value specified
on the command line. We can also use a less common name, given the
script calls it "flavour", let's use that.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>