[efi] Provide read-only access to EFI variables via settings mechanism

[efi] Provide read-only access to EFI variables via settings mechanism

EFI variables do not map neatly to the iPXE settings mechanism, since
the EFI variable identifier includes a namespace GUID that cannot
cleanly be supplied as part of a setting name.  Creating a new EFI
variable requires the variable's attributes to be specified, which
does not fit within iPXE's settings concept.

However, EFI variable names are generally unique even without the
namespace GUID, and EFI does provide a mechanism to iterate over all
existent variables.  We can therefore provide read-only access to EFI
variables by comparing only the names and ignoring the namespace
GUIDs.

Provide an "efi" settings block that implements this mechanism using a
syntax such as:

  echo Platform language is ${efi/PlatformLang:string}

  show efi/SecureBoot:int8

Settings are returned as raw binary values by default since an EFI
variable may contain boolean flags, integer values, ASCII strings,
UCS-2 strings, EFI device paths, X.509 certificates, or any other
arbitrary blob of data.

Signed-off-by: Michael Brown <mcb30@ipxe.org>