OvmfPkg/PlatformPei: use QemuFwCfgParseBool in UPDATE_BOOLEAN_PCD_FROM_...
    
    The UPDATE_BOOLEAN_PCD_FROM_FW_CFG() macro currently calls the
    module-private helper function GetNamedFwCfgBoolean(). Replace the latter
    with QemuFwCfgParseBool() from QemuFwCfgSimpleParserLib.
    
    This change is compatible with valid strings accepted previously.
    
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Per Sundstrom <per_sundstrom@yahoo.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2681
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>