x86-64/EFI: document building and usage

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

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+Building xen.efi requires gcc 4.5.x or above (4.6.x or newer recommended, as
+4.5.x was probably never really tested for this purpose) and binutils 2.22 or
+newer. Additionally, the binutils build must be configured to include support
+for the x86_64-pep emulation (i.e. --enable-targets=x86_64-pep or an option of
+equivalent effect should be passed to the configure script).
+
+Once built, "make install-xen" can place the resulting binary directly int
+the EFI boot partition, provided EFI_VENDOR is set (and EFI_MOUNTPOINT is
+overridden as needed, should the default of /boot/efi not match the target
+system).
+
+The binary itself will require a configuration file (names with the .efi
+extension of the binary's name replaced by .cfg, and - until an existing file
+is found - trailing name components dropped at '.', '-', and '_' separators
+will be tried) to be present in the same directory as the binary. (To
+illustrate the name handling, a binary named xen-4.2-unstable.efi would try
+xen-4.2-unstable.cfg, xen-4.2.cfg, xen-4.cfg, and xen.cfg in order.) One can
+override this with a command line option ("-cfg=<filename>").
+
+The configuration file consists of one or more sections headed by a section
+name enclosed in square brackets, with individual values specified in each
+section. A section named [global] is treated specially to allow certain
+settings to apply to all other sections (or to provide defaults for certain
+settings in case individual sections don't specify them). A typical file would
+thus look like this ('#' serving as comment character):
+
+**************************example begin******************************
+[global]
+default=sle11sp2
+
+[sle11sp2]
+options=console=vga,com1 com1=57600 loglvl=all noreboot
+kernel=vmlinuz-3.0.31-0.4-xen ignore_loglevel #earlyprintk=xen
+ramdisk=initrd-3.0.31-0.4-xen
+**************************example end********************************
+
+Other values to specify are
+
+video=gfx-<xres>[x<yres>[x<depth>]]
+(specifying a video mode to select if available; in case of problems the
+"-basevideo" command line option can be used to skip altering video modes)
+
+xsm=<filename>
+(specifying an XSM module to load)
+
+ucode=<filename>
+(specifying a CPU microcode blob to load)
+
+Filenames must be specified relative to the location of the EFI binary.
+
+Extra options to be passed to Xen can also be specified on the command line,
+following a "--" separator option.