bitbake.conf: set minimum required target kernel to 5.15

bitbake.conf: set minimum required target kernel to 5.15

In particular this enables a number of useful features in glibc
(which utilize newer kernel APIs), such as actually using 64 bit
time_t versions of kernel syscalls:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/kernel-features.h;h=07b440f4eea364b05fa49bf71ceebf78f80efe13;hb=HEAD#l164

In general, OLDEST_KERNEL setting is used in these two places:

- kernel.bbclass compares it with the target kernel version being built.
If a vendor BSP still offers an older kernel, OLDEST_KERNEL should be set to match.

- glibc recipe passes it as a parameter to the build so that additional features
and optimized paths that kernels older than OLDEST_KERNEL are enabled.

Note that there is a related setting, SDK_OLDEST_KERNEL, which remains as
it was (at 3.2.0) to ensure maximum compatibility with kernels on SDK host
machines; that setting is used to build nativesdk-glibc and verify the kernel
version when the SDK is being installed.

Build host kernel versions are not checked directly; compatible distros
are listed instead.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>