initscripts: fix bashism in bootmisc.sh

initscripts: fix bashism in bootmisc.sh

In the commit 'initscripts: save /etc/timestamp with seconds accuracy',
a bashism was introduced in the bootmisc.sh script in the code to set
the current date from the stored value in /etc/timestamp.  This causes
that operation to fail with the following message when /bin/sh is not
bash:

/etc/init.d/rc: /etc/rcS.d/S55bootmisc.sh: line 73: syntax error: bad substitution

Fixed by using pattern matching removal rather than bash-specific
substring expansion.

[YOCTO #6566]

(From OE-Core rev: c8a451adc71e4ce9ab963f61d7830c2d75aaffca)

Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>