binman: Allow collection to use entries from other sections

At present the collections etype only works with entries in the same
section. This can be limiting, since in some cases the data may be inside
a subsection, e.g. if there are alignment constraints.

Add a function to find the entries in an etype and have it search
recursively. Make use of this for mkimage also.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entries.rst b/tools/binman/entries.rst
index 3ee7b05..4dda34c 100644
--- a/tools/binman/entries.rst
+++ b/tools/binman/entries.rst
@@ -447,6 +447,9 @@
 base class for entry types which need to process data from elsewhere in
 the image, not necessarily child entries.
 
+The entries can generally be anywhere in the same image, even if they are in
+a different section from this entry.
+
 
 
 .. _etype_cros_ec_rw: