Rename library/ecp_alt.h to ecp_internal_alt.h

library/ecp_alt.h (declaring individual functions of the ECP module that can
be substituted, included when building the library with
MBEDTLS_ECP_INTERNAL_ALT enabled) clashes with ecp_alt.h (not provided,
declaring types of the ECP module when the whole implementation is
substituted, included when building the library with MBEDTLS_ECP_ALT enabled).
Depending on the search path during build, this can make MBEDTLS_ECP_ALT
unusable.

Rename library/ecp_alt.h to follow the naming convention of other alt headers:
MBEDTLS_XXX_ALT corresponds to xxx_alt.h.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog b/ChangeLog
index dc6e451..ddaf3fd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,7 +22,7 @@
      Various helpers and definitions available for use in alt implementations
      have been moved out of the include/ directory and into the library/
      directory. The files concerned are ecp_internal.h and rsa_internal.h
-     which have also been renamed to ecp_alt.h and rsa_alt_helpers.h
+     which have also been renamed to ecp_internal_alt.h and rsa_alt_helpers.h
      respectively.
    * Move internal headers.
      Header files that were only meant for the library's internal use and