Minor pw_bloat updates

This change renames the bloat_report template to pw_size_report as it is
more explicit.

Additionally, the option to override the global Bloaty config file on a
per-report basis is removed. This is due to Pigweed's policy of having
one target per build invocation. The Bloaty config is inherently tied to
an executable build target and linker script, both of which are provided
by the target. Overriding the Bloaty config without changing the target
can result in meaningless report output.

Change-Id: I1fa444d065d964a3a85ade50039e06ae807050f2
diff --git a/pw_bloat/bloat.rst b/pw_bloat/bloat.rst
index 3e72a1f..b6448b7 100644
--- a/pw_bloat/bloat.rst
+++ b/pw_bloat/bloat.rst
@@ -14,7 +14,7 @@
 
 Documentation integration
 =========================
-Bloat reports are easy to add to documentation files. All ``bloat_report``
+Bloat reports are easy to add to documentation files. All ``pw_size_report``
 targets output a ``.rst`` file containing a tabular report card. This file
 can be imported directly into a documentation file using the ``include``
 directive.