feat: introduce debug shell

Currently Servers time out while chain'ing to http://169.254.169.254/latest/user-data.
It is also not possible to ping them, as that results ins a Request timeout for icmp_seq.

I therefore introduced a short timeout for entering into the manual iPXE Shell for debugging the environment - this might be removed later on.

src/config/cloud/hetzner.ipxe

@@ -2,6 +2,16 @@
 
 echo Hetzner Cloud - iPXE boot via user-data
 echo CPU: ${cpuvendor} ${cpumodel}
+
+# Give user a chance to enter the shell
+prompt --key 0x02 --timeout 2000 Press Ctrl-B for the iPXE shell... || goto no_shell
+shell
+exit
+
+:no_shell
+
+# Perform automated system Boot
+
 ifstat ||
 dhcp ||
 route ||