libtomcrypt: fix headers installation path

libtomcrypt: fix headers installation path

libtomcrypt installs its headers by default in /usr/local/include under
the staging sysroot. This path is not in the default search patch of
some toolchains. This breaks the build of dropbear. Set the PREFIX make
variable to fix that.

While at it, split the long install command for better readability.

Fixes (dropbear):
http://autobuild.buildroot.net/results/215/2157679e276623ae875d1b31f3e5a68caf586536/
http://autobuild.buildroot.net/results/956/956d254675e6500c19c3bb7ccdf12ce136858720/
http://autobuild.buildroot.net/results/01e/01ec89a81c4ec6e36e2f81b5a9394050a91df654/

Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>