selftests/nolibc: fix up kernel parameters support

[ Upstream commit c388c992 ]

kernel parameters allow pass two types of strings, one type is like
'noapic', another type is like 'panic=5', the first type is passed as
arguments of the init program, the second type is passed as environment
variables of the init program.

when users pass kernel parameters like this:

    noapic NOLIBC_TEST=syscall

our nolibc-test program will use the test setting from argv[1] and
ignore the one from NOLIBC_TEST environment variable, and at last, it
will print the following line and ignore the whole test setting.

    Ignoring unknown test name 'noapic'

reversing the parsing order does solve the above issue:

    test = getenv("NOLIBC_TEST");
    if (test)
        test = argv[1];

but it still doesn't work with such kernel parameters (without
NOLIBC_TEST environment variable):

    noapic FOO=bar

To support all of the potential kernel parameters, let's verify the test
setting from both of argv[1] and NOLIBC_TEST environment variable.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Sasha Levin <sashal@kernel.org>