While this isn't likely a good practice, you can specify environment variables as kernel parameters. The Linux kernel documentation has some good information around it (important paragraph bolded):
The argument list
The kernel command line is parsed into a list of
strings (boot arguments) separated by spaces. Most of the boot
arguments have the form:
where 'name' is a unique keyword that is used to identify what part
of the kernel the associated values (if any) are to be given to. Note
the limit of 10 is real, as the present code handles only 10 comma
separated parameters per keyword. (However, you can reuse the same
keyword with up to an additional 10 parameters in unusually complicated situations, > assuming the setup function supports it.)
Most of the sorting is coded in the kernel source file init/main.c.
First, the kernel checks to see if the argument is any of the special
arguments 'root=', 'nfsroot=', 'nfsaddrs=', 'ro', 'rw', 'debug' or
'init'. The meaning of these special arguments is described below.
Then it walks a list of setup functions to see if the specified argument string
(such as 'foo') has been associated with a setup function
('foo_setup()') for a particular device or part of the kernel. If
you passed the kernel the line foo=3,4,5,6 then the kernel would
search the bootsetups array to see if 'foo' was registered. If it
was, then it would call the setup function associated with 'foo'
(foo_setup()) and hand it the arguments 3, 4, 5, and 6 as given on
the kernel command line.
Anything of the form 'foo=bar' that is not accepted as a setup function as
described above is then interpreted as an environment variable to be set.
A (useless?) example would be to use 'TERM=vt100' as
a boot argument.
Any remaining arguments that were not picked up by the kernel and
were not interpreted as environment variables are then passed onto
PID 1, which is usually the init(1) program. The most common argument
that is passed to the init process is the word 'single' which
instructs it to boot the computer in single user mode, and not launch
all the usual daemons. Check the manual page for the version of
init(1) installed on your system to see what arguments it accepts.
This AskUbuntu answer has some good instructions on how to modify your kernel boot parameters.