MIPS: bootm: Fix broken boot_env_legacy codepath

This patch fixes 2 bugs introduced by the following commit

2bb5b638 MIPS: bootm: rework and fix broken bootm code

The CONFIG_IS_ENABLED macro prepends 'CONFIG_' Hence, remove CONFIG_
from CONFIG_MIPS_BOOT_ENV_LEGACY usage.

Also, 2bb5b638 reworks bootm so that linux_env_legacy runs before
linux_cmdline_legacy. However, linux_env_legacy depends on
linux_cmdline_legacy running first as linux_cmdline_init initialilzes
linux_argp which linux_env_legacy later depends on during its
initialization.

Reorder the code so that linux_cmdline_legacy runs before
linux_env_legacy.

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>