RT4339: Fix handling of <internal/bn_conf.h>

The entire contents of <internal/bn_conf.h> are unwanted in the UEFI
build because we have to do it differently there. To support building
for both 32-bit and 64-bit platforms without re-running the OpenSSL
Configure script, the EDK2 environment defines THIRTY_TWO_BIT or
SIXTY_FOUR_BIT for itself according to the target platform.

The current setup is broken, though. It checks for OPENSSL_SYS_UEFI but
before it's actually defined, since opensslconf.h hasn't yet been
included.

Let's fix that by including opensslconf.h. And also let's move the
bn_conf.h doesn't even need to *exist* in the UEFI build environment.

This is also GH PR736.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>