[build] Replace random build ID with a deterministic one

[build] Replace random build ID with a deterministic one

Calculate the build ID as a checksum over the input files.  Since the
input files include $(BIN)/version.%.o which itself includes the build
target name (from which TGT_LD_FLAGS is calculated), this should be
sufficient to meet the requirement that the build ID be unique for
each $(BIN)/%.tmp even within the same build run.

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