arm64: kaslr: Refactor early init command line parsing

arm64: kaslr: Refactor early init command line parsing

Don't ask for *the* command line string to search for "nokaslr" in
kaslr_early_init(). Instead, tell a helper function to search all the
appropriate command line strings for "nokaslr" and return the result.

This paves the way for searching multiple command line strings without
having to concatenate the strings in early init.

Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Link: https://lore.kernel.org/r/20200921191557.350256-2-tyhicks@linux.microsoft.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>