common.mk: buildroot: enable top-level parallel build

Set BR2_PER_PACKAGE_DIRECTORIES=y to allow better parallelism when
building Buildroot packages [1] and therefore faster build times on
multi-core machines.

Link: [1] https://buildroot.org/downloads/manual/manual.html#top-level-parallel-build
Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/common.mk b/common.mk
index a5b92b9..eed9014 100644
--- a/common.mk
+++ b/common.mk
@@ -277,6 +277,7 @@
 DEFCONFIG_XEN_TOOLS=--br-defconfig build/br-ext/configs/xen_tools.conf
 endif
 
+BR2_PER_PACKAGE_DIRECTORIES ?= y
 BR2_PACKAGE_LIBOPENSSL ?= y
 BR2_PACKAGE_MMC_UTILS ?= y
 BR2_PACKAGE_OPENSSL ?= y