docs: Add a hint to clone pbs-stage2 into the correct directory.

In the previous step we switched into the kernel/linux.git directory.
Someone working through this manual step by step may unintentionally clone
the pbs into the kernel directory.

Change-Id: I8b966c79bf0c6e81f51d08329be5fdaa9f9e1d59
Signed-off-by: Hendrik Lippek <hendrik.lippek@avionic-design.de>
Reviewed-on: http://review.adnet.avionic-design.de/291
Reviewed-by: Soeren Grunewald <soeren.grunewald@avionic-design.de>
docs: Use correct target directory.

In this step of the manual we build the initial ramdisk, so the target must be:
build/initrd.

Change-Id: If4e5bed93585ab1f6896058deaa9a71d16392422
Signed-off-by: Hendrik Lippek <hendrik.lippek@avionic-design.de>
Reviewed-on: http://review.adnet.avionic-design.de/292
Reviewed-by: Soeren Grunewald <soeren.grunewald@avionic-design.de>