boards: arm: musca_b1: Add TFM support

This commit adds support for TF-M to the MUSCA B1.

When the CONFIG_BUILD_WITH_TFM flag is set, a secure and
non-secure processing environment image pair will be
generated, with the Zephy application image running on
the non-secure side.

The secure and non-secure binary images will be signed
for use with the BL2 secure bootloader.

Signed-off-by: Karl Zhang <karl.zhang@linaro.org>