target: Make TARGET_SERIAL independent of GRUB configuration

target: Make TARGET_SERIAL independent of GRUB configuration

GRUB_SERIAL is also used for the default serial on the target and not
only in grub. When no grub was build it was not available and the build
fails.

Rename GRUB_SERIAL to TARGET_SERIAL and make it always available on x86
and armsr targets.

Fixes: #14063
Fixes: b10768476f9d ("x86,armsr: interpolate GRUB_SERIAL into /etc/inittab")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>