boards: thingy91: add named THINGY91X_LAYOUT choice option

This allows to select Thingy:91 X layout using Kconfig.sysbuild default
option override.

Signed-off-by: Marcin Niestroj <[email protected]>

Author: mniestroj

boards/nordic/thingy91x/Kconfig.sysbuild

@@ -5,7 +5,7 @@ choice BOOTLOADER
 	default BOOTLOADER_MCUBOOT if THINGY91X_STATIC_PARTITIONS_FACTORY || THINGY91X_STATIC_PARTITIONS_NRF53_EXTERNAL_FLASH
 endchoice
 
-choice
+choice THINGY91X_LAYOUT
 	prompt "Pre-defined Thingy:91 X partition layout"
 	default THINGY91X_STATIC_PARTITIONS_FACTORY