boot/edk2: remove superfluous =TRUE in DEBUG_ON_SERIAL_PORT macro definition

Commit 8e9c8e624f "boot/edk2: introduce a new _OVMF_DEBUG_ON_SERIAL
option" adds a build macro definition with a "=TRUE" value.

edk2 OVMF readme suggests to define the "DEBUG_ON_SERIAL_PORT" macro
_without_ the "=TRUE" in [1].

The example build.sh script calls build to set the macro _with_
"=TRUE" in [2].

The "Build_Utility_Man_Page.rtf" documentation in [3] does not specify
either what exactly happen when a macro is defined without any value.

Looking at the "build.py" code in [4] shows that a macro definition of
the form "-DMYMACRO" is indeed equivalent as "-DMYMACRO=TRUE". This
was also tested with a qemu virt x86_64 grub2 EFI boot image, to
verify behaviors remain the same.

This commit removes the superfluous "=TRUE" as requested by Yann in [5].

Removing the "=TRUE" make it a bit clearer that it is the macro
definition itself that will enable the feature, rather than its
value. Defining, -DDEBUG_ON_SERIAL_PORT=FALSE (OFF, 0, or anything
else that would suggest the feature is disabled) would do the
opposite of the desired effect, and enable the feature.

[1] https://github.com/tianocore/edk2/blob/edk2-stable202305/OvmfPkg/README#L95
[2] https://github.com/tianocore/edk2/blob/edk2-stable202305/OvmfPkg/build.sh#L252
[3] https://github.com/tianocore/edk2/tree/edk2-stable202305/BaseTools/UserManuals
[4] https://github.com/tianocore/edk2/blob/edk2-stable202305/BaseTools/Source/Python/build/build.py#L2531
[5] https://lists.buildroot.org/pipermail/buildroot/2023-July/671059.html

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/edk2/edk2.mk

@@ -19,7 +19,7 @@ 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
+EDK2_BUILD_OPTS += -DDEBUG_ON_SERIAL_PORT
 endif
 else
 EDK2_BUILD_TYPE = RELEASE