[sw] Refactor exporting to only use BIN_DIR

We build code in the object directory (build-out, $OBJ_DIR), and then export it
into the bin directory (build-bin, $BIN_DIR). Within BIN_DIR, we
conventionally follow/mirror the directory structure in the source tree.

Before this patch we constructed the sub-directories $BIN_DIR/sw/host
and $BIN_DIR/sw/device in build_consts.sh, and passed them individually
to meson. After this patch, we only pass $BIN_DIR to meson, and let its
build logic figure out the rest.

This allows for a simplification in the meson logic without changing the
directory structure we produce.

The motivation for this change is the pending addition of the sw/otbn
directory, which would duplicate the same functionality for a third
time.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

meson_options.txt

diff --git a/meson_options.txt b/meson_options.txt
index c4feec5..75767b8 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -3,13 +3,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 option(
-  'dev_bin_dir',
-  type: 'string',
-  value: 'undef',
-)
-
-option(
-  'host_bin_dir',
+  'bin_dir',
   type: 'string',
   value: 'undef',
 )