Fix the boot process for Legacy BIOS machines

GitHub PR #235 adds an advanced boot mechanism for UEFI machines which
causes old Legacy BIOS nodes to hit the 3 GB RAM barrier in 32 Bit iPXE
very quickly. Therefore, detect if an EFI or Legacy BIOS is loaded and
fall back to the old boot path in the second case.

This commit fixes GitHub issue #497.

etc/ipxe/default.ipxe

@@ -19,6 +19,15 @@ echo Warewulf Controller: {{.Ipaddr}}
 echo Downloading Kernel Image:
 kernel --name kernel ${uri_base}&stage=kernel       || goto reboot
 
+# imgextract causes RAM space problems on non-EFI systems (because of the 3GB barrier
+# in 32-Bit mode).
+# -> Use the old initrd method with a compressed image to save as much RAM as possible
+# in this early boot stage.
+# See <https://github.com/hpcng/warewulf/issues/222> for more details.
+iseq ${platform} efi && goto efi || goto noefi
+
+:efi
+
 # try extracting compressed images first
 # NOTE: system overlay tends to be the smallest, so failure here is the cheapest
 echo Downloading Container Image:
@@ -56,6 +65,27 @@ echo Downloading Kernel Modules:
 initrd --name kmods ${uri_base}&stage=kmods             || goto reboot
 {{- end}}
 
+goto imoktogo
+
+:noefi
+
+echo
+echo Use legacy initrd mode with compressed images
+
+echo Downloading Container Image:
+initrd --name container ${uri_base}&stage=container&compress=gz || goto reboot
+
+echo Downloading System Overlay:
+initrd --name system ${uri_base}&stage=system&compress=gz       || goto reboot
+
+echo Downloading Runtime Overlay:
+initrd --name runtime ${uri_base}&stage=runtime&compress=gz     || goto reboot
+
+{{if ne .KernelOverride "" -}}
+echo Downloading Kernel Modules:
+initrd --name kmods ${uri_base}&stage=kmods&compress=gz         || goto reboot
+{{- end}}
+
 
 :imoktogo