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This patch adds capability to FVP to disable authentication dynamically
via the `disable_auth` property in TB_FW_CONFIG. Both BL1 and BL2 parses
the TB_FW_CONFIG for the `disable_auth` property and invokes the
`load_dyn_disable_auth()` API to disable authentication if the
property is set to 1. The DYN_DISABLE_AUTH is enabled by default for
FVP as it is a development platform. Note that the TB_FW_CONFIG has to
be authenticated by BL1 irrespective of these settings.

The arm_bl2_dyn_cfg_init() is now earlier in bl2_plat_preload_setup()
rather than in bl2_platform_setup() as we need to get the value of
`disable_auth` property prior to authentication of any image by BL2.

Change-Id: I734acd59572849793e5020ec44c6ac51f654a4d1
Signed-off-by: Soby Mathew <soby.mathew@arm.com>