tools: Add a function to obtain the size of a file

This will be used in mkimage when working out the required size of the FIT
based on the files to be placed into it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/mkimage.1 b/doc/mkimage.1
index b48f70b..1b9d18c 100644
--- a/doc/mkimage.1
+++ b/doc/mkimage.1
@@ -164,7 +164,7 @@
 .nf
 .B mkimage -f kernel.its -k /public/signing-keys -K u-boot.dtb \\\\
 .br
-.B -c "Kernel 3.8 image for production devices" kernel.itb
+.B -c """Kernel 3.8 image for production devices""" kernel.itb
 .fi
 
 .P
@@ -175,7 +175,7 @@
 .nf
 .B mkimage -F -k /secret/signing-keys -K u-boot.dtb \\\\
 .br
-.B -c "Kernel 3.8 image for production devices" kernel.itb
+.B -c """Kernel 3.8 image for production devices""" kernel.itb
 .fi
 
 .SH HOMEPAGE