config-desktop: install python3-yaml; surface parse_desktop_yaml.py errors

Two related fixes for BUILD_DESKTOP=yes on the configng-based desktop
selection path:

1. prepare-host.sh: add 'python3-yaml' to host_dependencies. configng's
   parse_desktop_yaml.py imports PyYAML, which neither the freshly-
   prepared docker container nor a from-scratch host had installed,
   so the parser bombed on every interactive desktop build with:

       ModuleNotFoundError: No module named 'yaml'

2. config-desktop.sh: stop swallowing the parser's stderr. The previous
   'python3 ... 2>/dev/null' meant any parser failure landed in the
   bash SUBSHELL ERR trap with zero diagnostic, e.g.:

       Error 1 occurred in SUBSHELL [ at config-desktop.sh:78 ]
       Error 1 occurred in main shell [ at config-desktop.sh:78 ]

   Capture stderr to a temp file, capture exit code via '|| de_rc=$?',
   and on non-zero invoke exit_with_error with the captured stderr
   inline so the actual cause shows up in the user's log on first run.
   The 'parser succeeded but returned empty list' branch is unchanged.

Author: igorpecovnik

lib/functions/configuration/config-desktop.sh

@@ -73,9 +73,25 @@ function interactive_desktop_main_configuration() {
 		local status_filter="supported"
 		[[ "${EXPERT}" == "yes" ]] && status_filter="supported,community"
 
-		local de_json
+		# Capture stdout and stderr separately and check the exit code
+		# explicitly. Previously stderr was redirected to /dev/null and
+		# only stdout was captured, so a non-zero exit (missing python3
+		# yaml module, malformed YAML, parser-side argparse change, …)
+		# left the SUBSHELL ERR trap firing at this line with no
+		# diagnostic — see the build hang at config-desktop.sh:78
+		# referenced from production logs.
+		local de_json de_stderr de_rc=0
+		de_stderr=$(mktemp)
 		de_json=$(python3 "${parser}" "${yaml_dir}" --list-json \
-			"${RELEASE}" "${ARCH}" --status "${status_filter}" 2>/dev/null)
+			"${RELEASE}" "${ARCH}" --status "${status_filter}" 2>"${de_stderr}") || de_rc=$?
+		if [[ "${de_rc}" -ne 0 ]]; then
+			local err_text
+			err_text=$(cat "${de_stderr}" 2> /dev/null || true)
+			rm -f "${de_stderr}"
+			exit_with_error "Desktop parser failed (exit ${de_rc}) for ${RELEASE}/${ARCH}" \
+				"stderr: ${err_text:-<empty>}"
+		fi
+		rm -f "${de_stderr}"
 		if [[ -z "${de_json}" || "${de_json}" == "[]" ]]; then
 			exit_with_error "No desktop environments available for ${RELEASE}/${ARCH}" \
 				"Parser returned an empty list"

lib/functions/host/prepare-host.sh

@@ -202,7 +202,9 @@ function adaptative_prepare_host_dependencies() {
 	### Python3 -- required for Armbian's Python tooling, and also for more recent u-boot builds. Needs 3.9+; ffi-dev is needed for some Python packages when the wheel is not prebuilt
 	### 'python3-setuptools' and 'python3-pyelftools' moved to requirements.txt to make sure build hosts use the same/latest versions of these tools.
 	### 'python3-dev' depends on distutils, so instead depend on libpython3-dev which doesn't.
-	host_dependencies+=("python3" "libpython3-dev" "libffi-dev")
+	### 'python3-yaml' is needed by configng's parse_desktop_yaml.py during
+	###   BUILD_DESKTOP=yes (config-desktop.sh::interactive_desktop_main_configuration).
+	host_dependencies+=("python3" "libpython3-dev" "libffi-dev" "python3-yaml")
 
 	# Needed for some u-boot's, lest "tools/mkeficapsule.c:21:10: fatal error: gnutls/gnutls.h"
 	host_dependencies+=("libgnutls28-dev")