kernel-uboot.bbclass: do not require the kernel build folder

kernel-uboot.bbclass: do not require the kernel build folder

The function must be executed in CWD. Make it more flexible by
specifying the kernel build folder as a parameter.

This is a refactoring without functional change. But later this change
will allow to use this function also with a kernel from the sstate-cache
instead of requiring the full kernel build folder structure.

Another preparation for using a kernel from sstate-cache is to persist
the linux_comp variable in a file next to the linux.bin file rather than
using a global shell variable.

This change also requires to adapt the kernel-uimage.bbclass
accordingly. This change also fixes a minor detail:
the kernel-uimage.bbclass used ${ instead of $ for evaluatiing a local
shell variable.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>