Support is added by:
tuned-2.4.1-1.20150601git.el7

Snapshot repo:
https://fedorapeople.org/~jskarvad/tuned/devel/repo/

Variables can be defined in [variables] section in profile configuration. Variables are specified the following way:

[variables]
VARIABLE1 = VALUE1
VARIABLE2 = VALUE2

Variables are expanded on definition and assignment. Previously defined variable is redefined by its new definition.

Variables can be referenced in plugins configuration by using '$' prefix, e.g.:

[variables]
delay = 20
   
[audio]
timeout = $delay
   
There can be also included external file with definition of variables by using 'include' directive:
    
[variables]
include = filename

The 'filename' is processed by ConfigObj parser. Comments are supported by '#' character. Multiple sections are not supported and files with multiple sections are flattened to single section configuration file.

The variables are also supported by the script plugin. It exports variables into environment before the script is executed. The variable names are prefixed by the 'TUNED_' string. Example:
   
[variables]
VARIABLE1 = VALUE1

will be exported into environment as:

TUNED_VARIABLE1 = VALUE1

If there is already TUNED_ prefix in the variable name, no other prefix is added, i.e.:

[variables]
TUNED_VARIABLE1 = VALUE1

will be exported as:

TUNED_VARIABLE1 = VALUE1

and not TUNED_TUNED_VARIABLE1.

tuned-2.4.1-1.20150602git.el7 changes relevant to this feature:

No more support for $VARIABLE syntax, now only the ${VARIABLE} syntax is supported. It is to resolve ambiguity and prepare room for the more complex syntax which will be introduced by built-in functions, e.g now the following works:

[variables]
delay = 20
   
[audio]
timeout = ${delay}

The timeout will be set to 20.

Also support for '$' was added, use $${VARIABLE} to disable expansion. The first '$' will be removet, e.g.:

[variables]
delay = 20
   
[audio]
timeout = $${delay}

The timeout will be set to '${delay}' literally which is not integer and error will be emitted.

Also undefined variables are not expanded to NULL (as in bash) but are taken literally, e.g.:

[audio]
timeout = ${delay}

The timout will be set to '${delay}' and the result will be the same as in the prevous example.

(In reply to Jaroslav Škarvada from comment #3)

> Also support for '$' was added

Should be:

Also support for escaping was added

Works now.  One thing I noted was that after applying the profile, grub.cfg reflected the new cmdline immediately, but /etc/tuned/bootcmdline took a few seconds to update.  Not a bug, just surprised me.

# rpm -q tuned
tuned-2.4.1-1.20150604git.el7.noarch
# grep cmdline tuned.conf 
cmdline=isolcpus=${isolated_cores} nohz_full=${isolated_cores} mce=ignore_ce intel_pstate=disable nosoftlockup
# cat variables.conf 
[variables]
# Examples:
# isolated_cores=2,4-7
isolated_cores=2-23
# tail -1 /etc/tuned/bootcmdline
TUNED_BOOT_CMDLINE="isolcpus=2-23 nohz_full=2-23 mce=ignore_ce intel_pstate=disable nosoftlockup"
# grep ^"set tuned" /boot/grub2/grub.cfg 
set tuned_params="isolcpus=2-23 nohz_full=2-23 mce=ignore_ce intel_pstate=disable nosoftlockup"

I had to change the syntax a bit to be consistent with the newly implemented feature, so since tuned build 20150605 quoting is done by backslash '\' not by doubling dollar '$', e.g.:

[variables]
a = 1
audio = a

[audio]
timeout = \${audio}

timeout will be set to '${audio}'.

but:

[variables]
audio = 1

[audio]
timeout = $${audio}

timeout will be set to '$1' which is not expanded further (there is only one level expansion)

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-2375.html