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>
(cherry picked from commit 73e1f9b0a46ccf55419f43c35ca762a8fdc3b32c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Baruch Siach