bootimg: Add grub-efi support

Create a new grub-efi.bbclass and integrate it into bootimg alongside the
syslinux support. This new class uses the output from the grub-efi-native
recipe. Thanks goes to Josef Ahmad <josef.ahmad@intel.com> for the original
build_grub_cfg() routine.

The EFI features are only added to the image if MACHINE_FEATURES contains
"efi". The resulting images are therefor either legacy boot only (like they
were originally) or legacy boot and EFI boot.

A new "dummy.bbclass" was added to allow for the conditional include
of grub-efi. This makes it so if efi support is not to be built in, we
don't spend time building grub-efi-native just because the include adds
the dependency.

There is a bug in the mkdosfs tool from the dosfstools package which causes
it to crash when the directory passed with the -d parameter contains
sub-directories. An /EFI/BOOT directory is required for a proper EFI
installation. Until it is fixed, we install to the top level directory
for the hddimg.

(From OE-Core rev: be95f54495bf9e03062f86b929c66cab6e385a03)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Josef Ahmad <josef.ahmad@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/dummy.bbclass

@@ -0,0 +1,2 @@
+# An empty bbclass to facilitate dynamic inherit, include,
+# and require statements.