build: Replace variant patterns with a list

Replace the variant patterns in the default-by-variant list with an
explicit list of matching BSPs.

The change was tested by comparing the output of

  ./waf bspdefaults

before and after the change.

spec/build/bsps/arm/imx/optcachedata.yml

@@ -9,7 +9,9 @@ default: true
 default-by-variant:
 - value: false
   variants:
-  - arm/.*qemu
+  - arm/lm3s6965_qemu
+  - arm/realview_pbx_a9_qemu
+  - arm/xilinx_zynq_a9_qemu
 description: |
   enable data cache
 enabled-by: true

spec/build/bsps/arm/imx/optcacheinst.yml

@@ -9,7 +9,9 @@ default: true
 default-by-variant:
 - value: false
   variants:
-  - arm/.*qemu
+  - arm/lm3s6965_qemu
+  - arm/realview_pbx_a9_qemu
+  - arm/xilinx_zynq_a9_qemu
 description: |
   enable instruction cache
 enabled-by: true