[gdb/testsuite] Add boards/README

Add a file gdb/testsuite/boards/README, to make it easier to get a high-level
overview of the various boards.

gdb/testsuite/boards/README

@@ -0,0 +1,88 @@
+This is a collection of boards for GDB.
+
+The file gdb/testsuite/boards/README contains an overview of the boards.  The
+idea is to group similar boards together, and have one line to describe a
+board.
+
+# Modify gdb default flags.
+readnow.exp
+	Run gdb with -readnow.
+
+# Post-process executable.
+cc-with-gdb-index.exp
+	Use gdb to create .gdb_index section.
+cc-with-debug-names.exp
+	Use gdb to create .debug_names section.
+cc-with-dwz.exp
+	Use dwz to compress debug info in executable.
+cc-with-dwz-m.exp
+	Use dwz to compress debug info in executable, optionally moving debug
+	info to a separate file.
+cc-with-gnu-debuglink.exp
+	Use strip to move the debug info from executable to a separate file,
+	and add a .gnu_debuglink to it.
+cc-with-tweaks.exp
+	Base board used by all boards in this section, but can be used by
+	itself,	f.i. do more than one type of post-processing.
+
+# Modify debug_flags.
+debug-types.exp
+	Add -fdebug-types-section.
+dwarf64.exp
+	Add -gdwarf64.
+gold.exp
+	Add -fuse-ld=gold.
+gold-gdb-index.exp
+	Add -fuse-ld=gold -Wl,--gdb-index -ggnu-pubnames
+stabs.exp
+	Add -gstabs+.
+
+# Test -gsplit-dwarf.
+fission.exp
+	Set debug_flags to -gdwarf-4 -gsplit-dwarf -ggnu-pubnames
+	-fdebug-types-section -Wl,--gdb-index -fuse-ld=gold.
+fission-dwp.exp
+	As fission.exp, but also post-process executables to generate a .dwp
+	file.
+
+# Test various things at the same time.
+dwarf4-gdb-index.exp
+	Set debug_flags to -gdwarf-4 -fdebug-types-section, and post-process
+	executable using gdb to create .gdb_index section.
+
+# [is_remote host] == 0 && [is_remote target] == 0.
+native-gdbserver.exp
+	Test gdbserver using gdb_protocol remote.
+native-extended-gdbserver.exp
+	Test gdbserver using gdb_protocol extended-remote.
+native-stdio-gdbserver.exp
+	Test gdbserver using stdio connection.
+
+# [is_remote host] == 0 && [is_remote target] == 1.
+remote-gdbserver-on-localhost.exp
+	Run gdbserver on remote target.
+remote-stdio-gdbserver.exp
+	Run gdbserver on remote target, using stdio connection.
+
+# [is_remote host] == 1 && [is_remote target] == 0.
+# Note: need to pass the board with --host_board instead of --target_board.
+local-remote-host-notty.exp
+	Run gdb on remote host.
+local-remote-host.exp
+	Run gdb on remote host in pseudo-terminal.
+
+# [is_remote host] == 1 && [is_remote target] == 1.
+# Note: need to pass the board with both --target_board and --host_board.
+local-remote-host-native.exp
+	Run gdb on remote host, run gdbserver on remote target, with
+	host == target.
+
+# Target-specific.
+simavr.exp
+	Use simavr as target.
+
+# Base boards, mostly to be included by other boards, so no individual
+# description.
+stdio-gdbserver-base.exp
+gdbserver-base.exp
+local-board.exp