Rename ecp_internal.h to ecp_alt.h

This gives it a more descriptive name and indicates to alt developers
that the definitions inside are not fully internal and are available
to alt developers for use.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index ac17ff1..55d7281 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -97,7 +97,7 @@
 #define mbedtls_free       free
 #endif
 
-#include "ecp_internal.h"
+#include "ecp_alt.h"
 
 #if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
 #if defined(MBEDTLS_HMAC_DRBG_C)