[Silabs] Fix enable_openthread_cli/uart build configuration

[jmartinez-silabs]

This pr fix an issue where enable_openthread_cli default value was defining some values and adding multiple uart files to the build when it was not needed for wifi ncp builds.

tested by building lighting app for thread and all 3 wifi ncp board supported and validating outputs

[github-actions]

PR #25835: Size comparison from ec2ad41 to 4fa4413

Increases (1 build for cc32xx)

platform	target	config	section	ec2ad41	4fa4413	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20309905	20309906	1	0.0

Full report (1 build for cc32xx)

platform	target	config	section	ec2ad41	4fa4413	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645745	645745	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301604	301604	0	0.0
			.debug_info	20309905	20309906	1	0.0
			.debug_line	2680953	2680953	0	0.0
			.debug_loc	2827596	2827596	0	0.0
			.debug_ranges	286376	286376	0	0.0
			.debug_str	3041114	3041114	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	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	380555	380555	0	0.0
			.symtab	257456	257456	0	0.0
			.text	537672	537672	0	0.0

[andy31415]

Need to replace the -1 magic value with something else. Either enum that is user-friendly or a bool with a default (I believe bool with default is clearer as "default/auto" in an enum seems to be ambigous)

[jmartinez-silabs]

@andy31415 Replying to your require changes comment.

I'll explain the situation here.

The default args depend on other possible configurations. True for a standard thread build, False for a build with wifi-ncp.
Since it depends on other build args that 1. are decided by the user, 2. need to be defined in another gni file elsewhere I cannot do the conditional default value in the declare_args since passed arguments value would not have replaced the default value yet et the gn flow.

I also want the user the have the final say in his build arguments. I need to know that the user did pass this argument in his build command and to not override what he passed with my conditional value. Hence the undefined value.

This doesn't apply, user error on my part!