linker script: fix keystore_pool size

Gnuk supports RSA keys up to 4096 bits. This require 1024 bytes of
storage (p and q are 256 bytes, n is 512 bytes). The linker script
should therefore reserve 1024 bytes per key instead of 512.

In practice it's not an issue at all as all supported MCU have a page
size bigger than 1024 bytes and Gnuk use one page per key.

src/gnuk.ld.in

@@ -135,11 +135,11 @@ SECTIONS
     {
         . = ALIGN (@FLASH_PAGE_SIZE@);
         _keystore_pool = .;
-        . += 512;
+        . += 1024;
         . = ALIGN(@FLASH_PAGE_SIZE@);
-        . += 512;
+        . += 1024;
         . = ALIGN(@FLASH_PAGE_SIZE@);
-        . += 512;
+        . += 1024;
         . = ALIGN(@FLASH_PAGE_SIZE@);
         _updatekey_store = .;
         . += 1024;