package/bootstrap: bump version to 3.3.7

package/bootstrap: bump version to 3.3.7

Extended _EXTRACT_CMDS and _INSTALL_TARGET_CMDS to handle updated
internal zip structure:

$ unzip -l ../dl/bootstrap-3.3.1-dist.zip
Archive:  ../dl/bootstrap-3.3.1-dist.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  2014-11-12 18:03   dist/
        0  2014-11-12 18:03   dist/css/
[...]

$ unzip -l ../dl/bootstrap-3.3.7-dist.zip
Archive:  ../dl/bootstrap-3.3.7-dist.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  2016-07-25 15:53   bootstrap-3.3.7-dist/css/
[...]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: better fix for the EXTRACT_CMDS, which consists in removing the
dist/ sub-directory.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>