kexecboot: apply OE specific fixes (pending upstream)

After 78ee4d8b1782445caecce8331e68efe83fc32044 in oe-core
the kernel is an *absolute* symlink in /boot.
Unfortunately this implies the stock images built with OE can not be booted.
The last patch of this commit fixes this issue.

The other patches are fixing build when S!=B, autotools-brokensep and
configure warnings.

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>