[nrf noup] boot/zephyr: fix and rename nrf54l15dk ext flash configs

nrf-squash! [nrf noup] boot/zephyr: update nrf54l15dk ext flash configs

This commit removes the partition definitions from nrf54L ext flash
configs, as they are no longer needed, as the pure DTS configuration
is no longer supported on nRF54L. They also caused issues, as they
were not compatible with nRF54L10 and nRF54L05.

It renames nrf54l15dk_nrf54l15_cpuapp_ext_flash.* overlays to
nrf54l15dk_ext_flash.*, as now the same configuration can be used
on nRF54L15, nRF54L10 and nRF54L05.

Signed-off-by: Artur Hadasz <[email protected]>

Author: ahasztag

boot/zephyr/boards/nrf54l15dk_nrf54l15_cpuapp_ext_flash.conf renamed to boot/zephyr/boards/nrf54l15dk_ext_flash.conf

@@ -0,0 +1,9 @@
+/ {
+	chosen {
+		nordic,pm-ext-flash = &mx25r64;
+	};
+};
+
+&mx25r64 {
+	status = "okay";
+};