build-setup: Refactor BITBAKE_CMD construction

Much of the BITBAKE_CMD is the same from one target to the next.
Factor out the things that are common.  This facilitates easier
access from other parts of the script to what is not common -
the layer directory for the target being built.

Remove a couple targets from the documentation that don't exist anymore.

Change-Id: Iab15d67f2fdf81ad49119c491228153c5b75ee63
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/build-setup.sh b/build-setup.sh
index d2ba32a..6c0b3db 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -34,7 +34,7 @@
 #                     fedora: 23|24|25
 #                     Default: "latest"
 #  target             The target we aim to build:
-#                     barreleye|evb-ast2500|firestone|garrison|palmetto|qemu
+#                     evb-ast2500|palmetto|qemu
 #                     romulus|s2600wf|witherspoon|zaius
 #                     Default: "qemu"
 #
@@ -132,31 +132,33 @@
 # Work out what build target we should be running and set BitBake command
 case ${target} in
   palmetto)
-    BITBAKE_CMD="TEMPLATECONF=meta-ibm/meta-palmetto/conf source oe-init-build-env"
+    LAYER_DIR="meta-ibm/meta-palmetto"
     ;;
   witherspoon)
-    BITBAKE_CMD="TEMPLATECONF=meta-ibm/meta-witherspoon/conf source oe-init-build-env"
+    LAYER_DIR="meta-ibm/meta-witherspoon"
     ;;
   evb-ast2500)
-    BITBAKE_CMD="TEMPLATECONF=meta-evb/meta-evb-aspeed/meta-evb-ast2500/conf source oe-init-build-env"
+    LAYER_DIR="meta-evb/meta-evb-aspeed/meta-evb-ast2500"
     ;;
   s2600wf)
-    BITBAKE_CMD="TEMPLATECONF=meta-intel/meta-s2600wf/conf source oe-init-build-env"
+    LAYER_DIR="meta-intel/meta-s2600wf"
     ;;
   zaius)
-    BITBAKE_CMD="TEMPLATECONF=meta-ingrasys/meta-zaius/conf source oe-init-build-env"
+    LAYER_DIR="meta-ingrasys/meta-zaius"
     ;;
   romulus)
-    BITBAKE_CMD="TEMPLATECONF=meta-ibm/meta-romulus/conf source oe-init-build-env"
+    LAYER_DIR="meta-ibm/meta-romulus"
     ;;
   qemu)
-    BITBAKE_CMD="source openbmc-env"
+    LAYER_DIR="meta-phosphor"
     ;;
   *)
     exit 1
     ;;
 esac
 
+BITBAKE_CMD="TEMPLATECONF=${LAYER_DIR}/conf source oe-init-build-env"
+
 # Configure Docker build
 if [[ "${distro}" == fedora ]];then