prepare p2 signing

Step1: no signing (sign=false) but prepare the usage of environment variables , which might come from the GH Action workflow later.
e.g. setting `P2_SIGN=true` env var enables the signing

Signed-off-by: Christoph Rueger <chrisrueger@gmail.com>

Author: chrisrueger

org.bndtools.p2/bnd.bnd

@@ -43,7 +43,26 @@ plugins: ${map;repo;${template;pluginnames;${@};latest}}
 
 -maven-release: 
 -buildrepo: 
--releaserepo: 
+-releaserepo:
+
+# P2 signing:
+# key (name of the key), 
+# passphrase (optional)
+# pubkey (read publickey file e.g. from ~./bnd/ folder)
+# below 
+# The idea is to first check global variables in ~./bnd/settings.json
+# and fallback to environment variables.  
+# For Github Actions you would set the environment variables.
+# fallback is "false" if none is set. 
+sign=${global;p2.sign;${env;P2_SIGN;false}}
+key=${global;p2.sign_key;${env;P2_SIGN_KEY}}
+passphrase=${global;p2.sign_passphrase;${env;P2_SIGN_PASSPHRASE}}
+pubkey="${global;p2.pub_key;${env;P2_PUB_KEY}}"
+ 
 -export features.bnd; \
     type=p2; \
-    name=org.bndtools.p2.jar
+    name=org.bndtools.p2.jar;\
+    sign=${sign};\
+    sign_key=${key};\
+    sign_passphrase=${passphrase};\
+    sign_pubkey=${pubkey};\