sys/hash/pbkdf2: Accept passwd as void * instead of uint8_t *

Having to cast a password provided as `const char *` to
`const uint8_t *` is a needless pain in the ass when using the API.
Hence, fix it by accepting passwords and salts as `const void *`
instead.

sys/hashes/pbkdf2.c

@@ -43,8 +43,8 @@ static void inplace_xor_digests(uint8_t *d1, const uint8_t *d2)
     }
 }
 
-void pbkdf2_sha256(const uint8_t *password, size_t password_len,
-                   const uint8_t *salt, size_t salt_len,
+void pbkdf2_sha256(const void *password, size_t password_len,
+                   const void *salt, size_t salt_len,
                    int iterations,
                    uint8_t *output)
 {

sys/include/hashes/pbkdf2.h

@@ -47,8 +47,8 @@ extern "C" {
  *                              recommended 10000
  * @param[out]  output          array of size PBKDF2_KEY_SIZE
  */
-void pbkdf2_sha256(const uint8_t *password, size_t password_len,
-                   const uint8_t *salt, size_t salt_len,
+void pbkdf2_sha256(const void *password, size_t password_len,
+                   const void *salt, size_t salt_len,
                    int iterations,
                    uint8_t *output);