treewide: fix uImage.FIT document paths

Commit 488445cefa ("doc: Move FIT into its own directory") moved the
documentation in doc/uImage.FIT to doc/usage/fit, subsequently all
documents and example sources have been converted to reStructuredText.

Fix (almost) all of the remaining occurrences of the old path and
filenames across the tree.

The exception is doc/uImage.FIT/command_syntax_extensions.txt which
apparently has been removed entirely, or at least I was unable to
locate where that document is now.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

lib/ecdsa/Kconfig

@@ -5,7 +5,7 @@ config ECDSA
 	  This enables the ECDSA (elliptic curve signature) algorithm for FIT
 	  image verification in U-Boot. The ECDSA algorithm is implemented
 	  using the driver model, so CONFIG_DM is required by this library.
-	  See doc/uImage.FIT/signature.txt for more details.
+	  See doc/usage/fit/signature.rst for more details.
 	  ECDSA is enabled for mkimage regardless of this option.
 
 if ECDSA

lib/rsa/Kconfig

@@ -6,7 +6,7 @@ config RSA
 	help
 	  RSA support. This enables the RSA algorithm used for FIT image
 	  verification in U-Boot.
-	  See doc/uImage.FIT/signature.txt for more details.
+	  See doc/usage/fit/signature.rst for more details.
 	  The Modular Exponentiation algorithm in RSA is implemented using
 	  driver model. So CONFIG_DM needs to be enabled by default for this
 	  library to function.
@@ -88,7 +88,7 @@ config RSA_SOFTWARE_EXP
 	  Enables driver for modular exponentiation in software. This is a RSA
 	  algorithm used in FIT image verification. It required RSA Key as
 	  input.
-	  See doc/uImage.FIT/signature.txt for more details.
+	  See doc/usage/fit/signature.rst for more details.
 
 config RSA_FREESCALE_EXP
 	bool "Enable RSA Modular Exponentiation with FSL crypto accelerator"