init-install-testfs: add grub serial line support

For automated hardware testing, boot process control
via serial interface is needed. As such, in grub, serial
line support is added upon testmaster image install.

Also add a specific timeout to automatically start
the master image upon start of testing phase.

Tested on multiple hardware targets without issues.

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
oe-core/init-install-testfs.sh: do not overwrite /etc/mtab if the link already exist

Overwriting of /etc/mtab would fail as below if the /etc/mtab link already
exist during installation phase, this patch fix this problem by checking
existance of the link before try to overwrite it.

Error message during installation if the /etc/mtab exists:
"cat: /proc/mounts: input file is output file"

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
init-install-testfs: fix typo

_EOF marker was not used properly
(space left before end of line).

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
init-install-testfs: create signature file for master image

Also small cosmetic changes.

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
meta-yocto-bsp: oeqa/controllers: add GrubTarget

add control for generic grub pc via serial line

Implementation [YOCTO #5615].

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>