rockchip: mkimage: rewrite padding calculation for SD/MMC and SPI images

In (first) breaking and (then) fixing the rkspi tool, I realised that
the calculation of the required padding (for the header-size and the
2K-in-every-4K SPI layout) was not as self-explainatory as it could
have been.  This change rewrites the code (using new, common functions
in rkcommon.c) and adds verbose in-line comments to ensure that we
won't fall into the same pit in the future...

Tested on the RK3399 (with has a boot0-style payload) with SD/MMC and SPI.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/rkcommon.c b/tools/rkcommon.c
index 5568b73..9595858 100644
--- a/tools/rkcommon.c
+++ b/tools/rkcommon.c
@@ -200,9 +200,13 @@
 	}
 }
 
-void rkcommon_vrec_header(struct image_tool_params *params,
-			  struct image_type_params *tparams)
+int rkcommon_vrec_header(struct image_tool_params *params,
+			 struct image_type_params *tparams,
+			 unsigned int alignment)
 {
+	unsigned int  unpadded_size;
+	unsigned int  padded_size;
+
 	/*
 	 * The SPL image looks as follows:
 	 *
@@ -228,4 +232,17 @@
 	/* Allocate, clear and install the header */
 	tparams->hdr = malloc(tparams->header_size);
 	memset(tparams->hdr, 0, tparams->header_size);
+	tparams->header_size = tparams->header_size;
+
+	/*
+	 * If someone passed in 0 for the alignment, we'd better handle
+	 * it correctly...
+	 */
+	if (!alignment)
+		alignment = 1;
+
+	unpadded_size = tparams->header_size + params->file_size;
+	padded_size = ROUND(unpadded_size, alignment);
+
+	return padded_size - unpadded_size;
 }