Fix Makefile syntax and provide default value for CONFIGURED_PLATFORM (…

…sonic-net#324)

Fix the assignment syntax for SECURE_UPGRADE_SIGNING_CERT. Also, provide
a default value for CONFIGURED_PLATFORM, since that's used in the call
to manage-config

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

Makefile

@@ -9,8 +9,9 @@ KERNEL_VERSION ?= 5.10.140
 KERNEL_SUBVERSION ?= 1
 kernel_procure_method ?= build
 CONFIGURED_ARCH ?= amd64
+CONFIGURED_PLATFORM ?= vs
 SECURE_UPGRADE_MODE ?=
-SECURE_UPGRADE_SIGNING_CERT =?
+SECURE_UPGRADE_SIGNING_CERT ?=
 
 LINUX_HEADER_COMMON = linux-headers-$(KVERSION_SHORT)-common_$(KERNEL_VERSION)-$(KERNEL_SUBVERSION)_all.deb
 LINUX_HEADER_AMD64 = linux-headers-$(KVERSION)_$(KERNEL_VERSION)-$(KERNEL_SUBVERSION)_$(CONFIGURED_ARCH).deb