Fix defconfig target for buildroot

Target doesn't have any dashes in kvm-unit-tests, i.e.,
qemu_aarch64_virt_kvmunittests_defconfig.

Signed-off-by: Fuad Tabba <tabba@google.com>
Change-Id: I25a14985b60344fdfd06c1610243abdf6906ea5b
diff --git a/Makefile b/Makefile
index 27339e9..105a114 100644
--- a/Makefile
+++ b/Makefile
@@ -138,7 +138,7 @@
 ## Buildroot
 ##
 
-BR_DEFCONFIG ?= qemu_aarch64_virt_kvm-unit-tests_defconfig
+BR_DEFCONFIG ?= qemu_aarch64_virt_kvmunittests_defconfig
 BR_VERBOSE ?= $(VERBOSE)
 
 BR_SRC := $(ROOT_DIR)/buildroot