[U-Boot] what means "extra-" versus "obj-" in Makefiles

[Robert P. J. Day]

  really a Kbuild question, i realize, but i'm looking at
common/Makefile, and i'm having trouble understanding the flipping
back and forth, as in:

  obj-$(CONFIG_ENV_IS_IN_DATAFLASH) += env_dataflash.o
  obj-$(CONFIG_ENV_IS_IN_EEPROM) += env_eeprom.o
  extra-$(CONFIG_ENV_IS_EMBEDDED) += env_embedded.o
  obj-$(CONFIG_ENV_IS_IN_EEPROM) += env_embedded.o
  extra-$(CONFIG_ENV_IS_IN_FLASH) += env_embedded.o
  obj-$(CONFIG_ENV_IS_IN_NVRAM) += env_embedded.o

what is the rationale for the jumping between "obj-" and "extra-"?
thanks.

rday

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
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