kbuild: clean up code duplication in cmd_fdtoverlay

kbuild: clean up code duplication in cmd_fdtoverlay

When resolving a merge conflict, Linus noticed the fdtoverlay command
duplication introduced by commit 49636c5680b9 ("kbuild: verify dtoverlay
files against schema"). He suggested a clean-up.

I eliminated the duplication and refactored the code a little further.

No functional changes are intended, except for the short logs.

The log will look as follows:

  $ make ARCH=arm64 defconfig dtbs_check
      [ snip ]
    DTC [C] arch/arm64/boot/dts/freescale/imx93-tqma9352-mba93xxca.dtb
    DTC [C] arch/arm64/boot/dts/freescale/imx93-tqma9352-mba93xxla.dtb
    DTC [C] arch/arm64/boot/dts/freescale/imx93-var-som-symphony.dtb
    DTC [C] arch/arm64/boot/dts/freescale/imx95-19x19-evk.dtb
    DTC     arch/arm64/boot/dts/freescale/imx8mm-venice-gw72xx-0x-imx219.dtbo
    OVL [C] arch/arm64/boot/dts/freescale/imx8mm-venice-gw72xx-0x-imx219.dtb

The tag [C] indicates that the schema check is executed.

Link: https://lore.kernel.org/lkml/CAHk-=wiF3yeWehcvqY-4X7WNb8n4yw_5t0H1CpEpKi7JMjaMfw@mail.gmail.com/#t
Requested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>