boot/edk2: introduce a new _OVMF_DEBUG_ON_SERIAL option

When EDK2 OVMF is built with debug, messages are printed to
IO port 0x402. Those messages are not shown in the normal
Qemu emulated serial port. Enabling this option will print
debug messages on the emulated serial port, potentially
mixing messages with UEFI serial console output.

See OVMF README:
https://github.com/tianocore/edk2/blob/master/OvmfPkg/README

This option has been useful to debug problems such as [1], in which the
emulator was hanging at startup without any log. Enabling this option
show the debug message on the console, before hanging:

    ...
    PlatformCpuCountBugCheck: Present=0 Possible=1
    PlatformCpuCountBugCheck: Broken CPU hotplug register block found. Update QEMU to version 8+, or
    PlatformCpuCountBugCheck: to a stable release with commit dab30fbef389 backported. Refer to
    PlatformCpuCountBugCheck: <https://bugzilla.tianocore.org/show_bug.cgi?id=4250>.
    ...

[1] http://lists.busybox.net/pipermail/buildroot/2023-July/670807.html

Signed-off-by: Julien Olivain <ju.o@free.fr>
Tested-by: Romain Naour <romain.naour@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

boot/edk2/Config.in

@@ -108,6 +108,21 @@ comment "QEMU SBSA depends on ATF not using EDK2 as BL33"
 
 endchoice
 
+config BR2_TARGET_EDK2_OVMF_DEBUG_ON_SERIAL
+	bool "OVMF Debug on Serial"
+	depends on BR2_ENABLE_DEBUG
+	depends on BR2_TARGET_EDK2_PLATFORM_OVMF_I386 || \
+		BR2_TARGET_EDK2_PLATFORM_OVMF_X64
+	help
+	  When EDK2 OVMF is built with debug, messages are printed to
+	  IO port 0x402. Those messages are not shown in the normal
+	  Qemu emulated serial port. Enabling this option will print
+	  debug messages on the emulated serial port, potentially
+	  mixing messages with UEFI serial console output.
+
+	  See OVMF README:
+	  https://github.com/tianocore/edk2/blob/master/OvmfPkg/README
+
 config BR2_TARGET_EDK2_FD_NAME
 	string
 	default "OVMF" if BR2_TARGET_EDK2_PLATFORM_OVMF_I386

boot/edk2/edk2.mk

@@ -16,8 +16,15 @@ EDK2_INSTALL_IMAGES = YES
 
 ifeq ($(BR2_ENABLE_DEBUG),y)
 EDK2_BUILD_TYPE = DEBUG
+ifeq ($(BR2_TARGET_EDK2_OVMF_DEBUG_ON_SERIAL),y)
+# DEBUG_ON_SERIAL_PORT is only tested to be set, so don't disable it, as
+# it would still be set.
+EDK2_BUILD_OPTS += -DDEBUG_ON_SERIAL_PORT=TRUE
+endif
 else
 EDK2_BUILD_TYPE = RELEASE
+# DEBUG_ON_SERIAL_PORT is only valid in debug builds, so useless to set
+# it (enabled or disabled) on a relase build.
 endif
 
 # Build system notes.