pk: use common header/footer macros for pkwrite and pkparse

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/pkparse.c b/library/pkparse.c
index 3bb5f7b..608c854 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -1534,8 +1534,7 @@
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     } else {
         ret = mbedtls_pem_read_buffer(&pem,
-                                      "-----BEGIN RSA PRIVATE KEY-----",
-                                      "-----END RSA PRIVATE KEY-----",
+                                      PEM_BEGIN_PRIVATE_KEY_RSA, PEM_END_PRIVATE_KEY_RSA,
                                       key, pwd, pwdlen, &len);
     }
 
@@ -1564,8 +1563,8 @@
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     } else {
         ret = mbedtls_pem_read_buffer(&pem,
-                                      "-----BEGIN EC PRIVATE KEY-----",
-                                      "-----END EC PRIVATE KEY-----",
+                                      PEM_BEGIN_PRIVATE_KEY_EC,
+                                      PEM_END_PRIVATE_KEY_EC,
                                       key, pwd, pwdlen, &len);
     }
     if (ret == 0) {
@@ -1594,8 +1593,7 @@
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     } else {
         ret = mbedtls_pem_read_buffer(&pem,
-                                      "-----BEGIN PRIVATE KEY-----",
-                                      "-----END PRIVATE KEY-----",
+                                      PEM_BEGIN_PRIVATE_KEY_PKCS8, PEM_END_PRIVATE_KEY_PKCS8,
                                       key, NULL, 0, &len);
     }
     if (ret == 0) {
@@ -1616,8 +1614,8 @@
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     } else {
         ret = mbedtls_pem_read_buffer(&pem,
-                                      "-----BEGIN ENCRYPTED PRIVATE KEY-----",
-                                      "-----END ENCRYPTED PRIVATE KEY-----",
+                                      PEM_BEGIN_ENCRYPTED_PRIVATE_KEY_PKCS8,
+                                      PEM_END_ENCRYPTED_PRIVATE_KEY_PKCS8,
                                       key, NULL, 0, &len);
     }
     if (ret == 0) {
@@ -1743,8 +1741,7 @@
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     } else {
         ret = mbedtls_pem_read_buffer(&pem,
-                                      "-----BEGIN RSA PUBLIC KEY-----",
-                                      "-----END RSA PUBLIC KEY-----",
+                                      PEM_BEGIN_PUBLIC_KEY_RSA, PEM_END_PUBLIC_KEY_RSA,
                                       key, NULL, 0, &len);
     }
 
@@ -1777,8 +1774,7 @@
         ret = MBEDTLS_ERR_PEM_NO_HEADER_FOOTER_PRESENT;
     } else {
         ret = mbedtls_pem_read_buffer(&pem,
-                                      "-----BEGIN PUBLIC KEY-----",
-                                      "-----END PUBLIC KEY-----",
+                                      PEM_BEGIN_PUBLIC_KEY, PEM_END_PUBLIC_KEY,
                                       key, NULL, 0, &len);
     }