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When CONFIG_I2C=m, CONFIG_I2C_SLAVE=y (bool), and CONFIG_IPMI_IPMB=y,
the build fails with:

ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_remove':
ipmi_ipmb.c:(.text+0x6b): undefined reference to `i2c_slave_unregister'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_thread':
ipmi_ipmb.c:(.text+0x2a4): undefined reference to `i2c_transfer'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_probe':
ipmi_ipmb.c:(.text+0x646): undefined reference to `i2c_slave_register'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_driver_init':
ipmi_ipmb.c:(.init.text+0xa): undefined reference to `i2c_register_driver'
ld: drivers/char/ipmi/ipmi_ipmb.o: in function `ipmi_ipmb_driver_exit':
ipmi_ipmb.c:(.exit.text+0x8): undefined reference to `i2c_del_driver'

This is due to having a tristate depending on a bool symbol.
By adding I2C (tristate) as a dependency, the desired dependencies
are met, causing IPMI_IPMB to be changed from =...