Allocate BSSL stack for SigningVerifier (#7291)
Allocate BSSL stack for SigningVerifier (#7291)

The BearSSL SigningVerifier was moved to the 2nd stack because some uses
required much more stack than available on the normal stack.

Add a reference to the second stack on object creation (which will
allocate it, if there is no BSSL stack already allocated), and delete
that reference on exit.

Fixes #7288
esp8266/Arduino,esp8266/Arduino,esp8266/Arduino,sticilface/Arduino,sticilface/Arduino,esp8266/Arduino,sticilface/Arduino,esp8266/Arduino,sticilface/Arduino,sticilface/Arduino