[Silabs] Fix a few issues preventing siwx917 platform builds.

[jmartinez-silabs]

Few build issues with the siwx917

• The software version string is in a common file for silabs platforms and is defined at build time
• missing variable definition.
• The EFR32 KeyValueStoreManagerImpl and MigrationManager implementation could and should be shared for the siwx917 platform. So move those files to the common platform folder and rename the class and namespace of the migration manager.

[github-actions]

PR #25557: Size comparison from 6bb8917 to 423c1ca

Decreases (1 build for cc32xx)

platform	target	config	section	6bb8917	423c1ca	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20267388	20267387	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	6bb8917	423c1ca	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	644425	644425	0	0.0
			(read/write)	203688	203688	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197088	197088	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	930235	930235	0	0.0
			.debug_aranges	87344	87344	0	0.0
			.debug_frame	300044	300044	0	0.0
			.debug_info	20267388	20267387	-1	-0.0
			.debug_line	2659771	2659771	0	0.0
			.debug_loc	2802807	2802807	0	0.0
			.debug_ranges	282960	282960	0	0.0
			.debug_str	3024017	3024017	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105929	105929	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	378514	378514	0	0.0
			.symtab	256624	256624	0	0.0
			.text	536372	536372	0	0.0