treewide: reduce append operators on SRC_URI

In a base bb file where we are setting the primary git repository, it
is rare that we need to `SRC_URI +=`.  This is an unnecessary pattern
that seems to have been copied throughout the repository.  Remove the
pattern where appropriate and simply set SRC_URI directly.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I430186a82f9582ba6196f5bf66b659af4092b48d
diff --git a/meta-phosphor/recipes-extended/stdplus/stdplus_git.bb b/meta-phosphor/recipes-extended/stdplus/stdplus_git.bb
index f3bde28..8a3c018 100644
--- a/meta-phosphor/recipes-extended/stdplus/stdplus_git.bb
+++ b/meta-phosphor/recipes-extended/stdplus/stdplus_git.bb
@@ -12,7 +12,7 @@
 PV = "0.1+git${SRCPV}"
 PR = "r1"
 
-SRC_URI += "git://github.com/openbmc/stdplus;branch=master;protocol=https"
+SRC_URI = "git://github.com/openbmc/stdplus;branch=master;protocol=https"
 
 S = "${WORKDIR}/git"