CI: test external-arm-toolchain with usrmerge enabled

We want to test meta-arm-toolchain layer with usrmerge enabled [1]
since it produce some breakage with current external ARM toolchains [2].

Instead of using a custom setting (poky + usrmerge enabled), use the
existing poky-altcfg provided by Yocto. poky-altcfg uses systemd as
init system and imply usermerge being enabled (new systemd v255
requirement) [3].

Note: It must be a 32bit machine, since there are currently no aarch64
host toolchains for aarch64 (some gitlab runner used by meta-arm are
aarch64 host) [4].

[1] https://docs.yoctoproject.org/scarthgap/ref-manual/features.html?highlight=usrmerge#distro-features
[2] https://lists.yoctoproject.org/g/meta-arm/message/5557
[3] https://git.openembedded.org/openembedded-core/commit/?id=802e853eeddf16d73db1900546cc5f045d1fb7ed
[4] 4bfa191ada

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Acked-by: Denys Dmytriyenko <denys@konsulko.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -295,7 +295,8 @@ qemuarm-secureboot:
         TOOLCHAINS: [gcc, clang]
         TCLIBC: [glibc, musl]
         TESTING: testimage
-      - TOOLCHAINS: external-gccarm
+      - DISTRO: [poky, poky-altcfg]
+        TOOLCHAINS: external-gccarm
         TESTING: testimage
       - KERNEL: linux-yocto-dev
         TESTING: testimage