tool: Use a different key to sign Non-secure image

This applies only to `ARM_MUSCA_B1` target
When MCUBOOT repo: UPSTREAM was set as default as part of TF-M 1.1
release, few things were changed:
MCUBOOT_IMAGE_NUMBER: '2'
MCUBOOT_UPGRADE_STRATEGY: 'OVERWRITE_ONLY'
MCUBOOT_HW_KEY: 'On'(default)

Therefore the signing strategy for Non-secure image (Mbed OS)
needs to be done with its own private key, which is validated during
boot.

Signed-off-by: Vikas Katariya <vikas.katariya@arm.com>

tools/targets/ARM_MUSCA_B1.py

@@ -71,7 +71,7 @@ def musca_tfm_bin(t_self, non_secure_bin, secure_bin):
     #2. Run imgtool to sign the non-secure mbed binary
     sign_args = Namespace(
         layout=image_macros_ns,
-        key=path_join(SCRIPT_DIR, 'musca_b1-root-rsa-3072.pem'),
+        key=path_join(SCRIPT_DIR, 'musca_b1-root-rsa-3072_1.pem'),
         public_key_format=None,
         align=1,
         dependencies=None,

tools/targets/musca_b1-root-rsa-3072.md

@@ -2,7 +2,7 @@
 
 A default RSA key pair is given to the Musca-B1 target.
 
-Public key was pre-compiled to `targets/TARGET_ARM_SSG/TARGET_MUSCA_B1/prebuilt/mcuboot.bin` and private key is in `musca_b1-root-rsa-3072.pem`.
+Public keys were pre-compiled to `targets/TARGET_ARM_SSG/TARGET_MUSCA_B1/prebuilt/mcuboot.bin` and private key is in `musca_b1-root-rsa-3072.pem` for Secure image and `musca_b1-root-rsa-3072_1.pem` for Non-Secure image.
 
 DO NOT use them in production code, they are exclusively for testing!
 

tools/targets/musca_b1-root-rsa-3072_1.pem

@@ -0,0 +1,39 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIIG5AIBAAKCAYEAv7ewn+jI0f4WHVOHl3kcFceZFmzKuC3Kwg1i+euP6ToYQ0fX
+u9VivOMzY6ejqFzzI3j9LQchH7lUcCipCNpQfp6OzGhOf0gN6ifoxu+tX51GSrxp
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+egyV0maL40d5fDsX2ZqCZQPrQ7+FhDHKg/jf3Z3lXHwTAKBNrQGN6g==
+-----END RSA PRIVATE KEY-----