package/qemu: refactor target emulator selection

The current mechanism to select emulation targets works this way:

- BR2_PACKAGE_QEMU_SYSTEM selects the "system" (softmmu) targets. It
  selects FDT and creates a dependency on the "dtc" package but this is
  not always necessary. Only 14 system targets, out of 31, actually
  require FDT.

- BR2_PACKAGE_QEMU_LINUX_USER selects the "linux-user" targets. It does
  not select FDT, which is not required by linux-user emulators.

- Alternatively, we fill BR2_PACKAGE_QEMU_CUSTOM_TARGETS with a list of
  emulators (e.g.  "x86_64-softmmu x86_64-linux-user"). Then we pass
  "--enable-system --enable-linux-user --target-list="..." to the
  configure script, so QEMU builds its list of default targets, from
  which it checks if the specified subset is valid.

Since CUSTOM_TARGETS does not select FDT, we can get build errors like
this:

  ../meson.build:2778:2: ERROR: Problem encountered: fdt not available but required by targets x86_64-softmmu

We could select FDT when CUSTOM_TARGETS is set, but this would force an
unnecessary dependency on dtc, as BR2_PACKAGE_QEMU_SYSTEM does.

In order to fix these problems, refactor the package configuration:

- Keep BR2_PACKAGE_QEMU_SYSTEM and BR2_PACKAGE_QEMU_LINUX_USER, which by
  default build all corresponding target emulators.

- Add a BR2_PACKAGE_QEMU_CHOOSE_TARGETS config, to permit choosing the
  desired emulators.

- Add configs for each supported target. They select FDT, when needed.

- Move QEMU to a separate menu, since the number of configuration itens
  became too large.

- Select BR2_LEGACY if BR2_PACKAGE_QEMU_CUSTOM_TARGETS is set, because
  this situation requires user intervention to reconfigure the package.

- Reorganize the make file accordingly. Selecting CHOOSE_TARGETS without
  choosing at least one emulator is considered an error.

Notes about the list of enabled targets:

- when the user enables both system and user emulation, we provide no
  way to be able to build all system bt only parts of user (or the other
  way around), because the qemu build system does not allow that: when a
  list of target is passed, it applies to both system and user
  emulation;

- as a consequence and in the same spirit, we also do not sanity-check
  that at least one system, resp. user, target is enabled when system,
  resp. user, emulation is enabled; we only require that one target is
  enabled, so that the list is not empty.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
[yann.morin.1998@free.fr:
  - always build at least user or system
  - add help text to BR2_PACKAGE_QEMU_CHOOSE_TARGETS
  - simplify prompt for BR2_PACKAGE_QEMU_CHOOSE_TARGETS
  - simplify list of targets with unique QEMU_TARGET_LIST_y
  - extend the commit log with "Notes about the list of enabled targets"
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

Config.in.legacy

@@ -146,6 +146,17 @@ endif
 
 comment "Legacy options removed in 2023.02"
 
+config BR2_PACKAGE_QEMU_CUSTOM_TARGETS
+	string "the QEMU specific targets option has been removed"
+	help
+	  This option has been replaced by a list of individual targets
+	  for the many architectures supported by QEMU.
+
+config BR2_PACKAGE_QEMU_CUSTOM_TARGETS_WRAP
+	bool
+	default y if BR2_PACKAGE_QEMU_CUSTOM_TARGETS != ""
+	select BR2_LEGACY
+
 config BR2_PACKAGE_XDRIVER_XF86_INPUT_KEYBOARD
 	bool "xf86-input-keyboard removed"
 	help