Build: Fixed compile error for the mbedtls-2.7.9 on Cygwin platform

When building Mbed TLS > 2.7 on Cygwin as part of the TF-M build,
related to a failure in the creation of a symbolic link.
Changes the build system to build Mbed TLS with CMAKE_HOST_UNIX = true.
More see Mbed TLS issue https://github.com/ARMmbed/mbedtls/issues/1496

Change-Id: I92c25fca7220e2b279e1c07b6c3f29f4161546e2
Co-Authored-By: Gyorgy Szing <Gyorgy.Szing@arm.com>
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>
1 file changed