arm64: cpufeatures: Fix handling of CONFIG_CMDLINE for idreg overrides

The built-in kernel commandline (CONFIG_CMDLINE) can be configured in
three different ways:

  1. CMDLINE_FORCE: Use CONFIG_CMDLINE instead of any bootloader args
  2. CMDLINE_EXTEND: Append the bootloader args to CONFIG_CMDLINE
  3. CMDLINE_FROM_BOOTLOADER: Only use CONFIG_CMDLINE if there aren't
     any bootloader args.

The early cmdline parsing to detect idreg overrides gets (2) and (3)
slightly wrong: in the case of (2) the bootloader args are parsed first
and in the case of (3) the CMDLINE is always parsed.

Fix these issues by moving the bootargs parsing out into a helper
function and following the same logic as that used by the EFI stub.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Fixes: 33200303

 ("arm64: cpufeature: Add an early command-line cpufeature override facility")
Link: https://lore.kernel.org/r/20210303134927.18975-2-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>