Build: Enable ninja-based builds

This commit enables to use of `-GNinja` for builds, which
provides significantly faster TF-M builds, and enables
building TF-M as a cmake external project with Zephyr's
build system.

```
$ cmake -GNinja \
  -DPROJ_CONFIG=`readlink -f ../configs/ConfigDefault.cmake` \
  -DTARGET_PLATFORM=LPC55S69 -DBL2=False -DCOMPILER=GNUARM ..
$ ninja
```

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>
Change-Id: I5deaaf9ffb3ec3329174881d70fe7033b531ec49