Config.in files: use if/endif instead of 'depends on' for main symbol

In the Config.in file of package foo, it often happens that there are other
symbols besides BR2_PACKAGE_FOO. Typically, these symbols only make sense
when foo itself is enabled. There are two ways to express this: with
    depends on BR2_PACKAGE_FOO
in each extra symbol, or with
    if BR2_PACKAGE_FOO
        ...
    endif
around the entire set of extra symbols.

The if/endif approach avoids the repetition of 'depends on' statements on
multiple symbols, so this is clearly preferred. But even when there is only
one extra symbol, if/endif is a more logical choice:
- it is future-proof for when extra symbols are added
- it allows to have just one strategy instead of two (less confusion)

This patch modifies the Config.in files accordingly.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>