linux-yocto: Fix COMPATIBLE_MACHINE regex match

With the current regex expression, a machine that is not part of the
compatible could match the regex expression.

For example, consider the following COMPATIBLE_MACHINE:

COMPATIBLE_MACHINE = "qemuarm|qemuarm64"

A machine definition bringing in "qemuarm-foo" would match against the
COMPATIBLE_MACHINE pattern above (see base.bbclass for implementation
details).

Fix this by matching the start and the end of the string.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>