efi_loader: helloworld.c: Explicitly use .rodata for loaded_image_guid

Commit bbf75dd9 ("efi_loader: output load options in helloworld")
introduced a const variable in efi_main() called loaded_image_guid which
got populated from a constant struct.

While you would usually expect a compiler to realize that this variable
should really just be a global pointer to .rodata, gcc disagrees and instead
puts it on the stack. Unfortunately in some implementations of gcc it does
so my calling memcpy() which we do not implement in our hello world
environment.

So let's explicitly move it to a global variable which in turn puts it in
.rodata reliably and gets rid of the memcpy().

Fixes: bbf75dd9 ("efi_loader: output load options in helloworld")
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>