[kdump] Fix kdump error message when a reboot is issued (sonic-net#7985)

dash doesn't support += operation to append to a variable's value. Use KDUMP_CMDLINE_APPEND="${KDUMP_CMDLINE_APPEND} " instead

The below error message is seen when a reboot is issued.

[ 342.439096] kdump-tools[13655]: /etc/init.d/kdump-tools: 117: /etc/default/kdump-tools: KDUMP_CMDLINE_APPEND+= panic=10 debug hpet=disable pcie_port=compat pci=nommconf sonic_platform=x86_64-accton_as7326_56x-r0: not found

files/image_config/kdump/kdump-tools

@@ -10,7 +10,7 @@ KDUMP_CMDLINE_APPEND="irqpoll nr_cpus=1 nousb systemd.unit=kdump-tools.service a
 # Disable advanced pcie features
 # Disable high precision event timer as on some platforms it is interfering with the kdump operation
 # Pass platform identifier string as part of crash kernel command line to be used by the reboot script during kdump
-KDUMP_CMDLINE_APPEND+=" panic=10 debug hpet=disable pcie_port=compat pci=nommconf sonic_platform=__PLATFORM__"
+KDUMP_CMDLINE_APPEND="${KDUMP_CMDLINE_APPEND} panic=10 debug hpet=disable pcie_port=compat pci=nommconf sonic_platform=__PLATFORM__"
 
 # Use SONiC reboot wrapper script present in /usr/local/bin post kdump
 PATH=/usr/local/bin:$PATH