[nrfconnect] Make flashing_script hex selection reliable

[mspang]

When building via

west build -b nrf52840dk_nrf52840 -t flashing_script

the wrong hex file is flashed, and doesn't boot. We need to flash zephyr/merged.hex, not zephyr/zephyr.hex.

The copy step attempts to do this, but does so incorrectly based upon a file existence test for which it has no declared dependency. This does not work reliably.

It's a bit tricky to figure out which file we need to use, but for nRF Connect there is a global variable ${merged_hex_to_flash} that suffices.

[github-actions]

PR #23003: Size comparison from dab3e8f to 4b56d9e

Increases (1 build for bl702)

platform	target	config	section	dab3e8f	4b56d9e	change	% change
bl702	lighting-app	bl702	(read/write)	1188027	1188043	16	0.0
			.debug_info	37902575	37902576	1	0.0
			.text	956624	956626	2	0.0

Full report (2 builds for bl702)

platform	target	config	section	dab3e8f	4b56d9e	change	% change
bl702	lighting-app	bl702		0	0	0	0.0
			(read only)	3262	3262	0	0.0
			(read/write)	1188027	1188043	16	0.0
			.bleromro	6296	6296	0	0.0
			.bleromrw	124	124	0	0.0
			.boot2	688	688	0	0.0
			.bss	67094	67094	0	0.0
			.bss_psram	29696	29696	0	0.0
			.comment	48	48	0	0.0
			.data	4272	4272	0	0.0
			.debug_abbrev	1506897	1506897	0	0.0
			.debug_aranges	133088	133088	0	0.0
			.debug_frame	486444	486444	0	0.0
			.debug_info	37902575	37902576	1	0.0
			.debug_line	5255713	5255713	0	0.0
			.debug_loc	3366207	3366207	0	0.0
			.debug_ranges	359136	359136	0	0.0
			.debug_str	3456572	3456572	0	0.0
			.hbn	509	509	0	0.0
			.hbn_noinit	260	260	0	0.0
			.init	342	342	0	0.0
			.init_array	144	144	0	0.0
			.psram	0	0	0	0.0
			.riscv.attributes	47	47	0	0.0
			.rodata	116536	116536	0	0.0
			.rsvd	3188	3188	0	0.0
			.shstrtab	293	293	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	564835	564835	0	0.0
			.symtab	171584	171584	0	0.0
			.tcm_data	36	36	0	0.0
			.tcmcode	3262	3262	0	0.0
			.text	956624	956626	2	0.0
		bl702+rpc		0	0	0	0.0
			(read only)	3262	3262	0	0.0
			(read/write)	1283963	1283963	0	0.0
			.bleromro	6296	6296	0	0.0
			.bleromrw	124	124	0	0.0
			.boot2	688	688	0	0.0
			.bss	75142	75142	0	0.0
			.bss_psram	29936	29936	0	0.0
			.comment	48	48	0	0.0
			.data	4800	4800	0	0.0
			.debug_abbrev	1644442	1644442	0	0.0
			.debug_aranges	140592	140592	0	0.0
			.debug_frame	511820	511820	0	0.0
			.debug_info	41809185	41809185	0	0.0
			.debug_line	5630248	5630248	0	0.0
			.debug_loc	3558839	3558839	0	0.0
			.debug_ranges	381592	381592	0	0.0
			.debug_str	3852472	3852472	0	0.0
			.hbn	509	509	0	0.0
			.hbn_noinit	260	260	0	0.0
			.init	342	342	0	0.0
			.init_array	160	160	0	0.0
			.psram	0	0	0	0.0
			.riscv.attributes	47	47	0	0.0
			.rodata	129928	129928	0	0.0
			.rsvd	3188	3188	0	0.0
			.shstrtab	293	293	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	624008	624008	0	0.0
			.symtab	189392	189392	0	0.0
			.tcm_data	36	36	0	0.0
			.tcmcode	3262	3262	0	0.0
			.text	1030326	1030326	0	0.0