[sw/host/mpsse] Move build of mpsse out of vendor tree.

Using vendor_hw.py script will not preserve our local files, such as
meson.build and README.md.  So move these files and the build out of
sw/host/vendor/mpsse into sw/host/mpsse, with changes to meson.build
so that it builds from the source files in ../vendor/mpsse/.
diff --git a/sw/host/meson.build b/sw/host/meson.build
index 10356b0..5317c6c 100644
--- a/sw/host/meson.build
+++ b/sw/host/meson.build
@@ -2,5 +2,5 @@
 # Licensed under the Apache License, Version 2.0, see LICENSE for details.
 # SPDX-License-Identifier: Apache-2.0
 
-subdir('vendor')
+subdir('mpsse')
 subdir('spiflash')
diff --git a/sw/host/vendor/mpsse/README.md b/sw/host/mpsse/README.md
similarity index 100%
rename from sw/host/vendor/mpsse/README.md
rename to sw/host/mpsse/README.md
diff --git a/sw/host/vendor/mpsse/meson.build b/sw/host/mpsse/meson.build
similarity index 87%
rename from sw/host/vendor/mpsse/meson.build
rename to sw/host/mpsse/meson.build
index 4f72d6f..8e9b435 100644
--- a/sw/host/vendor/mpsse/meson.build
+++ b/sw/host/mpsse/meson.build
@@ -6,8 +6,8 @@
   link_with: static_library(
     'mpsse',
     sources: [
-      'mpsse.c',
-      'support.c'
+      '../vendor/mpsse/mpsse.c',
+      '../vendor/mpsse/support.c'
     ],
     c_args: [
       '-I' + meson.source_root() + '/sw/host/vendor/mpsse'
diff --git a/sw/host/vendor/meson.build b/sw/host/vendor/meson.build
deleted file mode 100644
index be5c0b1..0000000
--- a/sw/host/vendor/meson.build
+++ /dev/null
@@ -1,5 +0,0 @@
-# Copyright lowRISC contributors.
-# Licensed under the Apache License, Version 2.0, see LICENSE for details.
-# SPDX-License-Identifier: Apache-2.0
-
-subdir('mpsse')