Let a unit carry a removelist and name the variables its conf script reads

[marcos-mendez]

The second half of the Keel decision 0010 prerequisite: the unit loop read overlay/ and conf and nothing else, so two thirds of what a component is could be expressed as a unit and the last third could not.

A removelist. A common component goes through three phases: its overlay, its conf script, its removelist. A unit had no slot for the third, so a component with a non-empty removelist could not leave the shared tree (common/removelists/mysql is 0 bytes, which is why the experiment did not notice). The loop now has that phase, after the unit conf scripts and before the common removelists, which is where the common removelists sit relative to the common conf scripts. Only a regular file counts, as for the product-local REMOVELIST.

CONF_VARS. _CONF_VARS is settled before any unit is looked at, so a unit had no way to ask for a build-time variable: mk/turnkey/mysql.mk line 1 is CONF_VARS += MYSQL_PASS, and the unit form of that line did not exist. A unit may now carry conf-vars, one variable name per line with # starting a comment; those names join CONF_VARS. Values still come from the environment or the product Makefile, exactly as a makefile fragment naming a variable leaves them. The names are read from UNIT_DIRS and not from UNITS, so the chroot environment of a layered build does not change with the set of units that build applies. An entry that is not a variable name stops make and names the file it came from, instead of reaching export as junk.

UNIT_DIRS moves above the CONF_VARS block, since _CONF_VARS now needs it.

Test plan

• tests/units.sh, 56 checks (was 38): the removelist applied and its position, a removelist that is a directory, a unit without one, a unit left out of UNITS, a failing removelist; for conf-vars a set variable reaching FAB_CHROOT_ENV, an unset one left out, two units contributing, the set not moving with UNITS, no file at all, and the error path with the offending file named.
• tests/coverage.sh 100: 75 of 75 checks, 100 percent, gate 100.
• tests/source-date-epoch.sh unchanged and passing.