This removal allows us to remove the `hdrs` target from every module; there
were only two modules that still used it, and they have both been fixed.

The purpose of this is to remove the dependency on tclx, a non-standard
package.

If a user runs the build from the E3_SITEMODS_PATH, then it can happen that
false dependencies get registered due to how the generated .d files are
parsed to determine module dependencies. Specifically, if you have header
file being used that is located at

${E3_SITEMODS_PATH}/e3-module/module/src/header.h

and it is _not_ being installed, then the dependency parsing script will
see a dependency as an absolute path, and so register that your module
depends on the module `e3-module` version `module`, which is obviously
not what was intended.

While we don't want to stricly _prevent_ a user from building there, it
seems wise to _warn_ a user that there may be unintended consequences.

The test rule for e3 modules actually does a cellinstall and run the
tests inside this temporary installation. The cellinstall needs to
overwrite some path variables to work this variables were not accessible
through the test rule as they are just changed if cell is on
MAKECMDGOALS. This made impossible to use the standard path variables
inside module's custom test rules.

To fix it the same mechanism used in cellinstall is implemented for test
and the variables paths are now overwritten. This makes possible ie. to
use E3_MODULES_INSTALL_LOCATION_BIN inside custom test rules.

This commit also introduces an unwanted behavior. As the test rule is
now overwriting the standard path variables it cannot longer be along
with the install rule. For example, running `make test install` will
install the module inside the test dir. This is a known issue for
cellinstall as well and a restructure of the Makefile code should be
made to isolate the rules.

This was needed since we had to have a way for the test suites to override this value. However,
this can also be acheived by writing to configure/CONFIG_CELL.local. This was added in a
refactoring of run_make to make more clear what variables can be passed there.

Local source mode can theoreticaly run 'make devbuild' and other related targets. However,
this yields somewhat undefined behaviour, since those will potentially be missing configuration
data or clashing with the regular build location. There is no reason to include dev targets for
local source mode, so they should be removed.