build: move br2 config files to a dedicated directory

br2/gpio

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+BR2_PACKAGE_LINUX_TOOLS=y
+BR2_PACKAGE_LINUX_TOOLS_GPIO=y

br2/kmscube

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+BR2_PACKAGE_KMSCUBE=y
+BR2_PACKAGE_MESA3D=y
+BR2_PACKAGE_MESA3D_OPENGL_EGL=y
+BR2_PACKAGE_MESA3D_OPENGL_ES=y
+
+# Software rasteriser.
+# Selects the following chain:
+# BR2_PACKAGE_MESA3D_DRIVER=y
+# BR2_PACKAGE_MESA3D_DRI_DRIVER=y
+BR2_PACKAGE_MESA3D_DRI_DRIVER_SWRAST=y

br2/qemu

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+BR2_PACKAGE_HOST_QEMU=y
+# False because otherwise we need the host to be as recent as guest, and the build fails with:
+# package/qemu/qemu.mk:110: *** "Refusing to build qemu-user: target Linux version newer than host's.".  Stop.
+BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE=n
+BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE=y
+BR2_PACKAGE_HOST_QEMU_VDE2=y

br2/x11

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+# X11
+# https://unix.stackexchange.com/questions/70931/how-to-install-x11-on-my-own-linux-buildroot-system/306116#306116
+BR2_PACKAGE_XAPP_TWM=y
+BR2_PACKAGE_XAPP_XCALC=y
+BR2_PACKAGE_XAPP_XCLOCK=y
+BR2_PACKAGE_XAPP_XEYES=y
+BR2_PACKAGE_XAPP_XINIT=y
+BR2_PACKAGE_XDRIVER_XF86_INPUT_KEYBOARD=y
+BR2_PACKAGE_XDRIVER_XF86_INPUT_MOUSE=y
+BR2_PACKAGE_XDRIVER_XF86_VIDEO_CIRRUS=y
+BR2_PACKAGE_XDRIVER_XF86_VIDEO_FBDEV=y
+BR2_PACKAGE_XDRIVER_XF86_VIDEO_VESA=y
+BR2_PACKAGE_XORG7=y
+BR2_PACKAGE_XSERVER_XORG_SERVER=y
+BR2_PACKAGE_XTERM=y
+BR2_TOOLCHAIN_BUILDROOT_CXX=y
+BR2_TOOLCHAIN_BUILDROOT_WCHAR=y