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This patch provides an API to access the authentication module that
will be used to verify the authenticity of the images loaded into
memory as part of the Trusted Board Boot process.

To include the authentication module as part of the build, set the
boolean build option TRUSTED_BOARD_BOOT. One single authentication
module must be registered at build time by setting the build option
AUTH_MOD=<mod_name>. All authentication modules will be located in
'common/auth/<mod_name>' and must present the <mod_name>.mk file that
will be included by the build system to compile the module sources.

To create an authentication module, an instance of auth_mod_t called
'auth_mod' must be declared in the module sources. The initialization
and verification functions provided by the module will be exported
through the function pointers specified when declaring this instance.

If an authentication module includes third party sources that do not
adhere to the C99 standard, the -pedantic option may be removed from
the build options by setting the flag DISABLE_PEDANTIC in the module
file <mod_name>.mk.

Change-Id: I080bb04bd421029bcdf22ec2c63807afbf061dcd