armbian-firstrun: use atomic write for armbianEnv.txt MAC randomization

Replace non-atomic sed -i with atomic write pattern (sed to temp file
-> chmod -> sync -> mv). Chain all steps with && so any failure
triggers cleanup of the temp file instead of silently keeping old
MAC values. Generate both MAC addresses before any I/O to avoid
partial state.

Author: mingzhangqun

packages/bsp/common/usr/lib/armbian/armbian-firstrun

@@ -60,12 +60,20 @@ case "$1" in
 	# get rid of purple background color in newt apps whiptail, partimage, debconf ... Reverting to Debian look.
 	[[ -f /etc/newt/palette ]] && sed -e 's/magenta/blue/g' -i /etc/newt/palette
 
-	# randomize mac in armbianEnv.txt
+	# randomize mac in armbianEnv.txt (atomic write to survive power loss)
 	if [[ -f /boot/armbianEnv.txt ]]; then
-		generate_random_mac
-		sed  -i "s/^ethaddr=.*/ethaddr=$MACADDR/" /boot/armbianEnv.txt
-		generate_random_mac
-		sed  -i "s/^eth1addr=.*/eth1addr=$MACADDR/" /boot/armbianEnv.txt
+		generate_random_mac; mac0="$MACADDR"
+		generate_random_mac; mac1="$MACADDR"
+		if sed -e "s/^ethaddr=.*/ethaddr=${mac0}/" \
+		       -e "s/^eth1addr=.*/eth1addr=${mac1}/" \
+		       /boot/armbianEnv.txt > /boot/armbianEnv.txt.tmp \
+		   && chmod --reference=/boot/armbianEnv.txt /boot/armbianEnv.txt.tmp \
+		   && sync /boot/armbianEnv.txt.tmp \
+		   && mv /boot/armbianEnv.txt.tmp /boot/armbianEnv.txt; then
+			:
+		else
+			rm -f /boot/armbianEnv.txt.tmp
+		fi
 	fi
 
 	# hardware workarounds per family