poky: Set INIT_MANAGER by a separate variable

Currently INIT_MANAGER cannot be overridden from local.conf. This is
complicated by the use of overrides in the various poky distro configs.
That is necessary, so use a separate variable to indirectly set the value
meaning local.conf can still override the result.

(From meta-yocto rev: da23d0c3edad0f5ee4cc4916276e214e8d53aad0)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/distro/poky-altcfg.conf

@@ -8,8 +8,8 @@ DISTRO = "poky-altcfg"
 DISTROOVERRIDES = "poky:poky-altcfg"
 #DISTROOVERRIDES = "poky:linuxstdbase"
 
-INIT_MANAGER:poky-altcfg = "systemd"
+POKY_INIT_MANAGER:poky-altcfg = "systemd"
 # systemd isn't suitable with musl
-INIT_MANAGER:poky-altcfg:libc-musl = "sysvinit"
+POKY_INIT_MANAGER:poky-altcfg:libc-musl = "sysvinit"
 
 require conf/distro/include/poky-distro-alt-test-config.inc

meta-poky/conf/distro/poky-tiny.conf

@@ -86,7 +86,7 @@ DISTRO_FEATURES:class-native = "${DISTRO_FEATURES_DEFAULT} ${POKY_DEFAULT_DISTRO
 DISTRO_FEATURES:class-nativesdk = "${DISTRO_FEATURES_DEFAULT} ${POKY_DEFAULT_DISTRO_FEATURES}"
 
 # enable mdev/busybox for init
-INIT_MANAGER:poky-tiny = "mdev-busybox"
+POKY_INIT_MANAGER:poky-tiny = "mdev-busybox"
 
 # FIXME: Consider adding "modules" to MACHINE_FEATURES and using that in
 # packagegroup-core-base to select modutils-initscripts or not.  Similar with "net" and

meta-poky/conf/distro/poky.conf

@@ -65,7 +65,8 @@ INHERIT += "uninative"
 BB_SIGNATURE_HANDLER ?= "OEEquivHash"
 BB_HASHSERVE ??= "auto"
 
-INIT_MANAGER = "sysvinit"
+POKY_INIT_MANAGER = "sysvinit"
+INIT_MANAGER ?= "${POKY_INIT_MANAGER}"
 
 # We need debug symbols so that SPDX license manifests for the kernel work
 KERNEL_EXTRA_FEATURES:append = " features/debug/debug-kernel.scc"