Support variable-size SPI chip for dummy programmer.

  flashrom -p dummy:emulate=VARIABLE_SIZE,image=image_file,size=4M

This is designed for firmware updater to preserve partitions easily. For example, the use case to keep VPDs when upgrading from H2O to H2C.

Change-Id: I84e1a793639cc33b769f6db41be0a0f0b1eacc78,,
BUG=Not a bug
TEST=Tested on Z600 and target.
# a H2C file is ready to write.
% flashrom -r H2O  # read existing content from BIOS, including VPDs.
% flashrom -p dummy:emulate=VARIABLE_SIZE,image=H2C,size=4M \
      -i RO_VPD -i RW_VPD -w H2O  # overwrite VPDs from H2O to H2C.
% flashrom -w H2C  # write it down, with VPDs from H2O.

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