build: remove shim related targets

build: remove shim related targets

There's no need to have shim specific targets, so just use the regular
xen makefile targets in order to build the shim binary.

When the shim is build as part of the firmware directory install the
stripped Xen binary to the firmware directory and place a binary with
symbols in the debug directory.

The objcopy step of the shim build is also removed in this patch:
since the shim is booted in PVH mode there's no need for the resulting
binary to be in elf32 format. Xen can load PVH kernels with either a
32 or 64bit elf header.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>