kernel: Fix path comparison in kernel staging dir symlinking

Due to an oversight in the do_symlink_kernsrc function, the path
comparison between "S" and "STAGING_KERNEL_DIR" is broken. The code
obtains both variables, but modifies the local copy of "S" before
comparing them, causing the comparison to always return false.

This can cause the build to fail when the EXTERNALSRC flag is enabled,
since the code will try to create a symlink even if one already exists.

This patch resolves the issue by comparing the variables before they are
modified.

Signed-off-by: Staffan Rydén <staffan.ryden@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>