mailpass from certgen

Closes #3 (closed)

!4 (merged) needs to be merged first.

Generates password for Turris smtp server dynamically with Certgen.

• Works on Turris 1.x, Omnia and MOX
• Uses turris/crypto-wrapper> to get a S/N
• Uses <S/N>@notify.turris.cz as an username
• Uses turris/sentinel/certgen> to get a password

Proposed turris/turris-os-packages> patch is:

diff --git a/cznic/user-notify/Makefile b/cznic/user-notify/Makefile
index 587bc78b8..0bc6dc5b1 100644
--- a/cznic/user-notify/Makefile
+++ b/cznic/user-notify/Makefile
@@ -22,7 +22,7 @@ include $(INCLUDE_DIR)/package.mk
 
 define Package/user-notify
        TITLE:=user-notify
-       DEPENDS:=+at +msmtp +ca-certificates +crypto-wrapper +create_notification
+       DEPENDS:=+at +msmtp +ca-certificates +crypto-wrapper +sentinel-certgen +create_notification
 endef
 
 define Build/Compile