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/pdbg/pdbg_3.6.bb b/meta-openpower/recipes-bsp/pdbg/pdbg_3.6.bb
index d0788b9..a14b92c 100644
--- a/meta-openpower/recipes-bsp/pdbg/pdbg_3.6.bb
+++ b/meta-openpower/recipes-bsp/pdbg/pdbg_3.6.bb
@@ -3,7 +3,7 @@
 LICENSE     = "Apache-2.0"
 LIC_FILES_CHKSUM = "file://${S}/COPYING;md5=3b83ef96387f14655fc854ddc3c6bd57"
 
-SRC_URI += "git://github.com/open-power/pdbg.git;branch=master;protocol=https"
+SRC_URI = "git://github.com/open-power/pdbg.git;branch=master;protocol=https"
 SRCREV = "3a21a79267bc5faa9e4bc56dc3124be610e9d475"
 
 DEPENDS += "dtc-native"