setup: Use machine-specific build directories by default

bitbake can struggle to cope with switching machine configurations in
the one build directory, so default to using machine-specific build
directories.

It's recommended that the following be added to your .profile
to provide a common location for bitbake data caches:

```
BB_ENV_EXTRAWHITE="SSTATE_DIR DL_DIR"
SSTATE_DIR="/var/cache/bitbake/sstate-cache"
DL_DIR="/var/cache/bitbake/downloads"
export BB_ENV_EXTRAWHITE SSTATE_DIR DL_DIR
```

Change-Id: I70abb6a6bc901c42266f92c9f8c230b793b7a0f9
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/setup b/setup
index 35a4a05..9676bb3 100755
--- a/setup
+++ b/setup
@@ -40,6 +40,7 @@
 		if [ -n "$target" ]; then
 			if [ "${name}" = "${target}" ]; then
 				echo "Machine ${target} found in ${tmpl%/conf}"
+				mkdir -p ${build_dir}
 				TEMPLATECONF="${tmpl}" source \
 						oe-init-build-env ${build_dir}
 
@@ -68,7 +69,7 @@
 else
 	bld_dir=$2
 	if [ -z "$2" ]; then
-		bld_dir="build"
+		bld_dir="build/$1"
 	fi
 	machine $1 $bld_dir
 fi