flex-builder: Document exported variables

Add a brief comment explaining that common variables are exported for use by
included .mk files, while machine-specific variables are exported by the
respective machine configs.

No functional change.

Signed-off-by: Luan Rafael Carneiro <luan.r@variscite.com>

Author: rafluan

tools/flex-builder

@@ -1159,6 +1159,8 @@ PKG_CONFIG_SYSROOT_DIR=$RFSDIR
 LD_LIBRARY_PATH=$DESTDIR/usr/lib:$RFSDIR/usr/lib:$RFSDIR/usr/lib/aarch64-linux-gnu
 PKG_CONFIG_PATH=$DESTDIR/usr/lib/pkgconfig:$RFSDIR/usr/lib/pkgconfig:$RFSDIR/usr/lib/aarch64-linux-gnu/pkgconfig:$RFSDIR/usr/share/pkgconfig
 
+# Export common variables so they are accessible in .mk files.
+# Additional machine specific variables are exported in the machine configuration files
 export ARCH DESTARCH CROSS_COMPILE FBDIR DESTDIR RFSDIR JOBS COMPONENT MACHINE KERNEL_DEVICETREE KERNEL_TREE \
        KERNEL_PATH KERNEL_OUTPUT_PATH INSTALL_MOD_PATH KERNEL_CFG BOOTTYPE MAKE CONFIGLIST MACHINE_LIST \
        CFGLISTYML BUILDARG DISTROTYPE DISTROVARIANT DISTROVARIANTOPT HOSTARCH SYSARCH SECURE \