linux-qcom-bootimg: Fix SyntaxWarning messages

Currently linux-qcom-bootimg.inc issues warnings like the following with
some versions of Python 3:
WARNING: linux-linaro-qcomlt-6.0-r0 do_qcom_img_deploy:
.../meta-qcom/recipes-kernel/linux/linux-qcom-bootimg.inc:51:
SyntaxWarning: "is" with a literal. Did you mean "=="?

Using is to test for equality is wrong. is only works "by accident" and
relies on some rather dubious assumptions about how CPython manages the
string literal pool

There are three occurrences, the first two are fixed the obvious way and
the final one is simply removed because the code was unreachable anyway
(the empty string already evaluates to False when used in a logic
expression).

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

recipes-kernel/linux/linux-qcom-bootimg.inc

@@ -48,9 +48,9 @@ python do_qcom_img_deploy() {
     output_sd_img =  os.path.join(qcom_deploy_dir, "boot-sd-%s.img" % (kernel_link_name))
 
     arch = d.getVar("ARCH")
-    if arch is "arm":
+    if arch == "arm":
         kernel_name = "zImage"
-    elif arch is "arm64":
+    elif arch == "arm64":
         kernel_name = "Image.gz"
     else:
         bb.fatal("Unuspported ARCH %s" % arch)
@@ -108,7 +108,7 @@ python do_qcom_img_deploy() {
                 shutil.copyfileobj(rfd, wfd)
 
         rootfs = getVarDTB("QCOM_BOOTIMG_ROOTFS")
-        if not rootfs or rootfs is "":
+        if not rootfs:
             bb.fatal("QCOM_BOOTIMG_ROOTFS is undefined")
 
         output = make_image("boot-%s-%s.img", rootfs)