scripts/runqemu: grep for line beginning with TMPDIR

scripts/runqemu: grep for line beginning with TMPDIR

Currently the grep regexp matches any occurance of
'TMPDIR=' but if you have another variable defined
e.g. OE_BUILD_TMPDIR=xxx then that gets picked up
too.

$ bitbake -e | grep TMPDIR=\"
TMPDIR="/home/kraj/work/angstrom/build/tmp-angstrom_2010_x-eglibc"
OE_BUILD_TMPDIR="/home/kraj/work/angstrom/build/tmp-angstrom_2010_x"

So we become a bit more stringent and look for
line starting with TMPDIR

$ bitbake -e | grep ^TMPDIR=\"
TMPDIR="/home/kraj/work/angstrom/build/tmp-angstrom_2010_x-eglibc"

make sure that it greps
only TMPDIR=xxx occurance and not values of other variables
whose names happens to end with TMPDIR

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>