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-openpower/recipes-bsp/ffs/ffs_git.bb b/meta-openpower/recipes-bsp/ffs/ffs_git.bb
index 0b40db0..fed5bf7 100644
--- a/meta-openpower/recipes-bsp/ffs/ffs_git.bb
+++ b/meta-openpower/recipes-bsp/ffs/ffs_git.bb
@@ -3,6 +3,6 @@
 PV = "v0.1.0+git${SRCPV}"
 
 SRCREV = "3ec70fbc458e32eef0d0b1de79688b4dc48cbd57"
-SRC_URI += "git://github.com/open-power/ffs.git;branch=master;protocol=https"
+SRC_URI = "git://github.com/open-power/ffs.git;branch=master;protocol=https"
 
 S = "${WORKDIR}/git"