dropbear: factorize the 'depends on BR2_PACKAGE_DROPBEAR'

Enclose all dropbear sub-options into a 'if BR2_PACKAGE_DROPBEAR'
... 'endif' block rather than having 'depends on BR2_PACKAGE_DROPBEAR'
for each option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>