package/libcurl: rename curl binary config symbol

Package optional or choice config symbols are usually prefixed with the
package config symbol name. Rename BR2_PACKAGE_CURL to
BR2_PACKAGE_LIBCURL_CURL to conform.

Update references to the old name.

Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>