new command "lf snoop" to snoop raw ADC values

fpga/lo_read.v (lf_field): new argument.
fpga/fpga_lf.v: modify accordingly.

armsrc/apps.h (FPGA_MAJOR_MODE_LF_READER): Rename as FPGA_MAJOR_MODE_LF_ADC.
armsrc/apps.h (FPGA_LF_ADC_READER_FIELD): New LF option.
armsrc/lfops.c: Modify accordingly.

client/cmdlf.c (CmdLFSnoop): New command.
armsrc/appmain.c, armsrc/lfops.c, client/cmdlf.h, include/usb_cmd.h: Modify accordingly.