Import gcrypt public-key cryptography and implement signature checking.

grub-core/lib/libgcrypt/mpi/generic/mpi-asm-defs.h

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+/* This file defines some basic constants for the MPI machinery.  We
+ * need to define the types on a per-CPU basis, so it is done with
+ * this file here.  */
+#define BYTES_PER_MPI_LIMB  (SIZEOF_UNSIGNED_LONG)
+
+
+
+
+
+