sdk: fix relocate symlink failed

Install SDK to non-default dir, sysmlink mkfs.vfat is invalid

$ ./sdk.sh -y -d ./dnf-2 -S -D
$ ls sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat -al
lrwxrwxrwx 1 hjia users 99 Aug 10 20:38 sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat -> /opt/windriver/wrlinux-graphics/21.32/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat.dosfstools

Since commit [bc4ee545 sdk: Decouple default install path from
built in path] applied, sdk relocates symlink failed, it should
replace $SDK_BUILD_PATH rather than $DEFAULT_INSTALL_DIR, just
like above commit did

Without this commit:
...
|+ for l in $($SUDO_EXEC find $native_sysroot -type l)
|++ readlink path-to/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat
|++ sed -e s:/usr/local/oecore-x86_64:path-to:
|+ ln -sfn /opt/windriver/wrlinux-graphics/21.32/sysroots/x86_64-wrlinuxsdk-linux/
usr/bin/mkfs.vfat.dosfstools path-to//sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat
...

After appling this commit:
...
|+ for l in $($SUDO_EXEC find $native_sysroot -type l)
|++ readlink path-to/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat
|++ sed -e s:/opt/windriver/wrlinux-graphics/21.31:path-to:
|+ ln -sfn path-to/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat.dosfstools
path-to/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/mkfs.vfat
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>