Make MBEDTLS_MPI_IS_PUBLIC thumb friendly

In Thumb instructions, constant can be:

- any constant that can be produced by shifting an 8-bit value left by any
  number of bits within a 32-bit word
- any constant of the form 0x00XY00XY
- any constant of the form 0xXY00XY00
- any constant of the form 0xXYXYXYXY.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index ee69aa7..6c214a5 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -108,7 +108,7 @@
  *  }
  * not the other way round, in order to prevent misuse. (This is, if a value
  * other than the two below is passed, default to the safe path.) */
-#define MBEDTLS_MPI_IS_PUBLIC  0x2a2a
+#define MBEDTLS_MPI_IS_PUBLIC  0x2a2a2a2a
 #define MBEDTLS_MPI_IS_SECRET  0
 
 /** Count leading zero bits in a given integer.