security: Add default stack protection level

STACK_CANARIES was enabling canaries in all functions using the compiler
flag -fstack-protector-all. This became confuse with the addition of the
options STRONG and EXPLICIT.

This commit adds the missing option (default level) and disambiguous the
options mapping them close to the compiler flags.

Now we have the following options:

STACK_CANARIES            -> fstack-protector
STACK_CANARIES_STRONG     -> fstack-protector-strong
STACK_CANARIES_ALL        -> fstack-protector-all
STACK_CANARIES_EXPLICIT   -> fstack-protector-explicit

Note that from now on STACK_CANARIES_ALL is the symbol that adds canaries
for all functions.

Signed-off-by: Flavio Ceolin <flavio.ceolin@gmail.com>