mk: fix bsd module build

mk: fix bsd module build

When building shared libs (for both GCC and CLANG targets), -fPIC flag
has been added to CFLAGS and leaks to BSD module build system causing
the following error:

fatal error: error in backend: Cannot select: 0x802ad8010: i64 = X86ISD::WrapperRIP 0x802ade110
      [ID=13]
  0x802ade110: i64 = TargetGlobalAddress<i8** @__stack_chk_guard> 0 [TF=5] [ID=10]

Reset CFLAGS to MODULE_CFLAGS before building BSD module.

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>